if you think about it he wants to pay half of the amount he paid for last month so its basically a trial and error/ guess and check question so i started with 15 and went 15+30 because half of 30 is 15 so you would multiply by 2 so 15+30= 45+60, 60 being half of 30 45+60= 105+120, again half is 60 105+120= 225 once i found out 15 was not correct i then tried the number 10. 10+20= 30+40=70+80= 150
