If Mrs. Smith lives to age 88 (she is now retiring at 65 years) what annual APR will result in the 420000 reaching a balance of 1000000 upon her death, if she deposits them in an account that offers monthly compounding?
suppressing all the irrelevant zeros, we have
42(1+r/12)^(12*23) = 100
(1+r/12)^276 = 2.3809
1+r/12 = 1.003148
r = 3.777%