let

x = the present age of Alex

y = the present age of Precy

in 3 years Alex will be three times as old as his sister Precy

x + 3 = 3 (y + 3)

A year ago, Alex was seven times as old as Precy

x - 1 = 7*(y - 1)

by solving the system of equations

x + 3 = 3 (y + 3)

x - 1 = 7*(y - 1)

we find

x = 15 years

y = 3 years

Alex is 15 years old now.

Percy is 3 years old now.