Question

There are b boys in the class.  This is three more than four times the number of girls.  How many girls are in the class?​

Answer 1

no. of boys = b

no. of girls = 3 more than 4 times of boys

= 3 more than 4xb or 4b

= 4b + 3

Answer 2

let x represent the unknown number that you are asked to find:  Let x be the number of girls.

(Although b is not known -- it is an arbitrary constant -- it is not what you are asked to find.)

The problem states that "This" -- b -- is three more than four times x:

 4x + 3 = b.  

 Therefore,

 4x = b − 3  

 

x = b − 3

  4 .

The solution here is not a number, because it will depend on the value of b.  This is a type of "literal" equation, which is very common in algebra.