If the growth is g per quarter, then after 1 quarter it would grow to 40 x (1+g). After 2 quarters it would be 40 x (1+g)^2, and after 3 quarters it would be 40 x (1+g)^3.
After three quarters of growth is was 60, so 40 x (1+g)^3 = 60.
Don’t worry about learning the formula. The rate of growth in this question was not needed to get the marks – just the fact that it had grown was all the was relevant.