The average length of four sides is= 15. Now imagine 4 points in the number line, equidistantly placed with the step "d", which is the common difference of the AP. Then the distance from the center to the fourth (right-most) point is 1.5d. So, 1.5d = 24 - 15 = 9. Hence, d = 6. Thus the sides are 24, 24-6 = 18, 18 - 6 = 12 and 12 - 6 = 6. Answer. The sides are 6, 12, 18 and 24 cm. Check. 6 + 12 + 18 + 24 = 60 cm.