Clavis usuræ: or, A key to interest, both simple and compound: containing practical rules ... whereby all the various cases of interest ... may very easily be resolv'd, both by the pen and a small table of logarithms ... To which is added, rules to be observ'd in estimating the value of annuities, or leases, and insurances for lives, Pc. Also, the business of rebate or discompt, and the equation of payments ...