There are 21 boys in math class. If 75% of the class are girls, how many kids are in the class?

Question

I originally said 49 students because if you do 21/x+75/100 and cross multiply you get 28 and i added 21 and 28 together but what is the right answer and steps

Answer 1

Dear Becca,    

                   

      Question:  How many students are in the class?

 

     Percents are based on 100%,   so if there are 75%  girls, then 100-75=25% = percent of boys.

 

75%= 75/100= 3/4

 

25% = 25/100 =1/4

 

Therefore, 1/4 of some number of students in the class = 21

 

Let X= number of students in the class

 

Then (1/4) X = 21                   X/4  =21/1                   Cross multiply to find "X."

 

Once you find  the value for 'X," you can check your work by substituting into the following equations:

 

(1/4)X=21   number of boys                   and  (3/4) X=__________ number of girls

 

                                                                    Susan C.

 

Answer 2

If 75% of the class are girls, then 100% - 75% = 25% are boys, so:

 

(Number of Students in the Class)*(0.25) = 21

 

(Number of Students in the Class) = 21/(0.25) = _______?

 

---------------------------------

 

If you want to use a proportion:

 

 25%        21

--------- = -----

100%        N

 

Where N is the Number of Students in the class.  Cross multiplying, we get:

 

25N = 100*21

 

N = 100*21/25 = ______?

 

Try it both ways and see if you get the same answer.