You a Miguel Cervantes de Navas y Colon, captain in the Royal Spanish Army in Seville in the year 1842. Outside your barracks window is a stack of cannonballs, as shown in the illustration. On an idle afternoon you decide to calculate the number of cannonballs in the stack. what is the number of cannonballs.

650 cannonballs Explanation step by step: upper layer = 1 ^ 2 second layer = 2 ^ 2 third layer = 3 ^ 2 layer 12 is 12 ^ 2 The formula is: n * (n + 1) * (2n + 1) / 6 n = 12 12 (12 + 1) (2 * 12 + 1) / 6 12 * 13 * 25/6 = 650 There are 650 cannonballs in this pyramid.