Just create a problem and figure it out.
Let's say the original diameter is 4. This means the radius is 4/2 = 2.
A = 2πr^2
A = 2π(2)^2
A = 2π(4)
A = 8π
The new diameter is 50% larger than the original.
50% of 4 is 2. 2 + 4 = 6. So the new diameter is 6. The radius is 6/2 = 3.
A = 2πr^2
A = 2π(3)^2
A = 2π(9)
A = 18π
Old area was 8π, new area is 18π. Now divide the difference from the original area:
18π - 8π = 10π
10π/8π = 1.25 * 100 = 125%