If x= side of the square, and A = area of square, then
A = x^2
dA/dx = 2x, or
dA = 2x*dx, where dx is the differential in x (same as error)
dA/A = 2x*dx/(x^2) = 2*dx/x = 2*(0.05cm)/(15cm) = 0.00667 = 0.667%
for b):
if dA/A = 0.025 = 2*dx/x, then
dx/x=(0.025)/2 = 0.0125 = 1.25 %
I hope I helped. :)