Increasing the length by 10% means we multiply the old length x by 1.1 since
100% + 10% = 1+0.1 = 1.1
Old length = x
New length = 1.1x
Old area = x*x = x^2
New area = (1.1x)*(1.1x) = (1.1*1.1)*(x*x) = 1.21x^2
So the area increases by 21%
1.21 = 1+0.21 = 100% + 21%
