first of all the miniutes is not a linar function cause the first miniutes is 45 the 12 minuets later 57 but 11 minuets later 68 minuets so the minuets is not linear
what i did to find the credit was that i took 24.15-1.59 wich gave me 22.59 then i did 22.59-1.59 which gave me 21.03 if that is not correct