72 rule, formulated as T = 720, is a quick way to estimate the time T for a portfolio to double its initial amount given a fixed level of compounded yield r. For example, if the yearly interest rate is 7%, it will take approximately Tz ~ 10 years for a saving account to double its balance. 72 100 x 0.07 In 2 0.69 The equation (1+r)= 2 implies the time required to double the initial invested amount is T When r is small, recall the In(1+r) In(1+r) relation ln(1 + r) ar 12/2 = r(r r/2) in calculus. Show that when r~ 8%, T = 72 100r