The computer Jack needs to buy is selling for </body>,200.00. He only have 65% of the cost. How much more will Jack need to make the purchase? The cost is 1200. He has 0.65 of 1200. So, (0.65)(1200) = 780. Jack has 0. Let m = the amount if money Jack needs to buy the computer. m = 00 - 0 m = 0. Jack needs 0 to purchase the computer.