Two unlike signs automatically means to just subtract. So, now it looks like 8-12. Subtract the smaller number from the larger number, 12-8, which equals to four. But keep the sign of the larger value, which was 12, and twelve had a negative sign in he beginning, so 8+(-12) = -4.
First you want to change the 20% into a decimal which is .20 Next you want to multiply 90 by .20 because that will give you the answer for 20% of 90 90x.20=18
