I put 160, but the answer was 156 ^^ I didn't express it as ar^n-1, everyone I know messed it up (5 in my class of 8 are 2s, 3s and 4s and only 3 of us are predicted 6). We had double it because the ball comes up and down. What was the answer to part b) again, I remember putting 27 or 28 can't remember which.