there are 32 balls which are red , blue or green . if 15 balls are green and the sum of red balls and green balls is less than 23 , at most how many red balls are there ?
"r + g + b = 32 g = 15 r + g < 23 = > r + 15 < 23 = > r < 8 = > at most 7 red balls answer : d"
subtract(15, const_1)