15. Since p=215. 851-p would be 851-215 which equals 636
18. Since a=6 and b=5 and c=3, ab/2 would be 6x5/2 which equals to 30/2. Now, you must add 30/2 to 4c. And again since c=3, it would be 4x3 which equals to 12. So now the equation would be 30/2+12. You must now turn the fraction into a whole number by 30÷2 which is 15. The question would now ask 15+12 so the answer would be 27