Question 1143742
How many boys are there... 

{{{matrix(1,3,B,""="","?")}}}

...in a group of 27 students... 

{{{matrix(1,3,B+G,""="",27)}}}

...if there are four-fifths as many girls as there are boys?

{{{matrix(1,3,G,""="",expr(4/5)B)}}}<pre>
Substitute {{{expr(4/5)B}}} for G in

{{{matrix(1,3,B+G,""="",27)}}}

{{{matrix(1,3,B+expr(4/5)B,""="",27)}}}

Multiply every term on both sides by the LCD, which is 5:

{{{matrix(1,3,5*B+5*expr(4/5)B,""="",5*27)}}}

{{{matrix(1,3,5B+cross(5)expr(4/cross(5))B,""="",5*27)}}}

{{{matrix(1,3,5B+4B,""="",135)}}}

{{{matrix(1,3,9B,""="",135)}}}

Divide both sides by 9:

{{{matrix(1,3,9B/9,""="",135/9)}}}

{{{matrix(1,3,B,""="",15)}}}

There are 15 boys.

Edwin</pre>