To find the present value of a Survivorship Annuity of £1 on the life of A to be entered on at the death of B, supposing the purchaser to effect an increasing insurance on A's life so that the sum insured may correspond with the amount of his outlay (of purchase money, premiums, and interest) at the end of each year up to the average duration of the joint lives of A and B, and to be at the end of that period of the exact amount which the annuity purchased would cover the annual interest thereon, beside providing for the premiums to keep it insured during the remainder of A's life.