Question 562871
Let
Present age of Sue = s
Present age of Emma = e
Present age of Abby = a

Given
Sue is twice as old as Emma 
s=2e
s/2=e
e=s/2.........(1)
and 7 years older than Abby
s=a+7
s-7=a
a=s-7........(2)
The sum of their ages is 53
a+e+s=53.......(3)


We have three unknowns and three equations.


Put the values of e and a from (1) and (2) to (3)
a+e+s=53.......(3)
s-7+s/2+s=53
Multiply by 2 both sides of the equation
2s-14+s+2s=106
5s-14=106
5s=106+14
5s=120
5s/5=120/5
s=24
Put the value of s in (1)
e=s/2.........(1)
e=24/2
e=12
Put the value of s in (2)
a=s-7........(2)
a=24-7
a=17



Present age of Sue = s  = 24 years old 
Present age of Emma = e = 12 years old
Present age of Abby = a = 17 years old