The formula for the area of a circle is $\pi r^2$ So in other words, the area depends on radius-squared Assuming the two semicircles are similar fractions of the total circle, the ratio between the two areas should be the ratio between the radii squared: So AreaR/AreaS = (r/s)^2 AreaR/75pi = (3/5)^2 AreaR = 75pi * 9/25 AreaR = 27pi