8 weeks the coat would cost 150 * (1-0.15+ = 127.50
9 week it would cost 127.50 *(1-0.05) = 121.13
10 week = 121.13 * (1-0.05) = 115.07
week 11 = 115.05 * (1-0.05) = 109.32
he can buy it in week 11, because it would be less than 110 and the sore would still make a profit, because it costs more than 100