Let's define
x: money earned the first week
y: money earned the second week
z: money earned the third week
He earned a total of $168, that is,
x + y + z = 168
Each week his earnings doubled, then
y = 2x
z = 2y = 2(2x) = 4x
Substituting these equations into the first one, we get:
x + 2x + 4x = 168
7x = 168
x = 168/7
x = 24
and
y = 2*24 = 48
z = 4*24 = 96
He earned $24 the first week, $48 the second week, and $96 the third week
