Rodney is 3 times as old as his sister. A year ago, he was 4 times as old. How old are Rodney and his sister now?
s = sister's age now
3s = Rodney's age now
s-1 = sister's age a year ago
3s-1 = Rodney's age a year ago
so
3s-1 = 4 (s-1)
3s - 1 = 4s - 4
I think you can take it from there.