Question 526700
let m = Mark's present age
let s = sister's present age
:
Write an equation for each statement:
:
"Mark is 11 years older than his sister."
m = s+11
:
"In 8 years he will be twice as old as she will be then."
m + 8 = 2(s+8) 
m + 8 = 2s + 16
m = 2s + 16 - 8
m = 2s + 8
Replace m with (s+11), from the 1st statement
s + 11 = 2s + 8
11 - 8 = 2s - s
3 = s
s = 3 yrs is the sisters age
then
3 + 11 = 14 is Marks age
:
:
See if the checks out in the statement:
"In 8 years he will be twice as old as she will be then."
14 + 8 = 2(3+8)
22 = 2(11)