Let Tim be t and let Susan be s
t + s = 20 ........... .(i)
In 4 years time Tim will be = t + 4 and Susan will be = s + 4
Tim's will be 3/4 of Susan's
t + 4 = (3/4)( s + 4) ........(ii)
From (i) t = 20 -s, Substitute this into (ii)
20 -s + 4 = (3/4)(s + 4)
24 - s = (3/4)(s +4)
4*(24 - s) = 3*(s + 4)
96 - 4s = 3s + 12
-4s - 3s = 12 - 96
-7s = - 84
s = -84/-7
s = 12
Susan is 12 years old now.