Because the formula is something like baseXhight divided by two. and 2 times sixty is 120 so 120/12=10 that divided by 2 is five so the answer is either 5 or 10
The formula for this specific equation would be base times height divided by two. and 2 times 60 is 120 so 120/12=10, with this answer being that divided by 2 which would be five so the answer is either 10
