The sum of the ages of tim and susan is 20 years. In 4 years, Tim's age will be three-fourths of Susan's age. How old is Susan now

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.