Lesson 15 Section 2 Parts, pluralLet that long rectangle represent 3; and let us divide it into thirds, that is, into three equal parts; and let us shade one of them  1 out of 3. 1 is one third of 3. Two of the three rectangles have now been shaded. 2 out of 3. But each rectangle is a third part of the whole. Therefore those two rectangles together are two third parts of the whole. Or simply two thirds. Those words, "two thirds," are to be taken literally, like two apples or two chairs. One third is the unit. We are counting thirds: One Third, two Thirds. 1, 2. Now, 2 is not a part of 3, because 3 is not a multiple of 2. We say it is parts of 3, plural. 2 is two third parts of 3. Notice how each number  2 out of 3  says its name. "Two thirds." In Spanish, they say dos terceras partes  two third parts  which is literally true. But in English, unfortunately, we are not used to saying that. 

 
100 is not a multiple of 75. Therefore we say that 75 is parts of 100. It is three fourth parts, or simply three fourths. If the entire figure represents 15, then 5 is the third part of 15, and 10 is two third parts of 15. If the entire figure represents 18, then 6 is the third part of 18, and 12 is two third parts of 18. If the entire figure represents 21, then 7 is the third part of 21, and 14 is two third parts of 21. And so on. Two thirds of a number are twice as much as one third. Example 1. Fifths. Here is 10 divided into fifths, that is, into five equal parts: Each Fifth  each 2  is a unit, and we will count them. 2 is is the fifth part of 10. 4 is two fifth parts of 10. 6 is three fifth parts of 10. 8 is four fifth parts of 10. And 10 is all five of its fifth parts. 



Example 2. How much is two thirds of 12? Answer. To name two thirds, we must first name one third. Say: "One third of 12 is 4. Therefore, two thirds are 8." Two thirds of 12, then, are equivalent to 8 out of 12. Example 3. How much is three fourths, or three quarters, of 28? Answer. Three fourths are three times more than one fourth. Begin: "One fourth of 28 is 7. So, three fourths are three 7's: 21. We can illustrate this with any number that has a fourth part, namely any multiple of 4. For example, 12, 40, 100: If 12 is divided into fourths  that is, into 3's  then each 3 is a fourth part of 12. 6 is two fourth parts of 12. 9 is three fourth parts of 12. Count those Fourths If 40 is divided into its fourths, then 10 is the fourth part of 40. 20 is two fourths (also half) of 40. 30 is three fourths of 40. Finally, if 100 is divided into its fourths, then 25 is one fourth of 100. 50 is two fourths or half of 100. And 75 is three fourths of 100. Example 4. How much is four fifths of 15? Answer. To name four fifths, we must first name one fifth. One fifth of 15 is 3. Each 3 is a fifth part of 15. 6 is two fifth parts of 15. 9 is three fifth parts of 15. 12 is four fifth parts, or simply four fifths, of 15. Example 5. In a class of 32 students, five eighths are girls. How many boys are there? Solution. The whole class is eight eighths. Therefore, if five eighths are girls, then the remaining three eighths are boys. Now, one eighth of 32 is 4. Therefore three eighths are three times 4: 12. There are 12 boys in the class. Percent: Parts of 100% We have seen that a percent is another way of expressing a part of a number. Since 100% is the whole (Lesson 4, Question 5), and since 50% is half of 100%, then 50% means half. 50% of 40  half of 40  is 20. Since 25% is a quarter of 100%, then 25% is another way of saying a quarter. A quarter of 40  25% of 40  is 10. Since 75% is three quarters of 100%, then 75% means three quarters. 30 is 75% of 40. Whichever part or parts the percent is of 100%, the percent means that part or those parts. Fifths Let the circle represent 100%, and let us divide it into fifths, that is, into five equal parts  into 20's. Each 20% is a fifth of the circle. Two fifths of the circle  of 100%  is 40% Three fifths is 60%. Four fifths is 80%. That is what those percents mean: 20% means one fifth. 40% means two fifths. 60% means three fifths. 80% means four fifths. 100% is the whole; it is all five fifths. Example 6. How much is 60% of 45? Answer. 60% means three fifths. To name three fifths, you must first name one fifth. One fifth of 45 is 9. Therefore three fifths are three 9's: 27. Example 7. A scarf that sells for $35 is on sale at 40% off. How much do you pay? Answer. "40% off" means that two fifths of the price will be subtracted. One fifth of 35 is 7. Therefore, two fifths are 14. You will pay $14 less: $35 − $14 = $21. More simply, since 40% of the price will be subtracted, then you will pay 60%. 60%  or three fifths of $35  is three times $7, which is $21. For more on percent, see Lesson 17. Finally, we will state this theorem:
(Euclid, VII.4.) We will illustrate that with each number less than 9. We will see that each number less than 9 is either a part of 9 or parts of 9. Now, 9 units can be divided either into Ninths or Thirds: (If 9 is divided into Ninths, then it is divided into 1's. If 9 is divided into Thirds, then it is divided into 3's: 3, 6, 9.) Let us now see how each number is related to 9. 1 is the ninth part  or one ninth  of 9. 2 is two ninth parts of 9. (The point again is that each 1 is a ninth part of 9.) 3 is three ninths of 9  and also the third part of 9. 4 is four ninths of 9. Count them! 5 is five ninths of 9. 6 is six ninths  and also two thirds  of 9. 3 + 3. 7 is seven ninths of 9. 8 is eight ninths of 9. Notice again how each number says its name: 1 is one ninth of 9. 2 is two ninths of 9. 3 is three ninths of 9. The first number says its cardinal name. 9 says its ordinal name. Each number less than 9, then, is either a part of 9 or parts of it. We can therefore express in words how each number is related to 9. We can say that 5, for example, is "five ninths" of 9. Example 8. What relationship has 9 to 10? Answer. If we divide 10 into 1's, then each 1 is a tenth part of 10. 1 is one tenth of 10. 2 is two tenths of 10. 3 is three tenths of 10. And so on, until we come to 9: 9 is nine tenths of 10. Again, the numbers 9 and 10 say their names. 9 says its cardinal name "nine." 10 says its ordinal name "tenth." Please "turn" the page and do some Problems. or Continue on to the next Lesson. Introduction  Home  Table of Contents Please make a donation to keep TheMathPage online. Copyright © 2014 Lawrence Spector Questions or comments? Email: themathpage@nyc.rr.com 