<PRE><B>Question 304076</B>
let the children in the family be x

their sum of ages = y

let the parents sum of ages be z

z=2y
.
FIVE YEARS AGO
z-10 = 4(y-5x)
z-10=4y-20x
but z=2y

plug the value of z 

2y-10=4y-20x

2y-20x=-10------------ A
.
.

15 YEARS LATER
z+30 = y+15x

2y+30=y+15x

y-15x=-30-------------B

2(y-15x)-(2y-20x)= 2*-30 - 2*-10

2y-30x-2y+20x=-60+20

-10x=-40

x=4

there were 4 children


</PRE>