Again, like the last post I responded too....please double check my answer but it should be D.

We are told that there is exactly the same amount of chairs on each row. To find out how many rows we have, we subtract 1 from the total of chairs.

Number 1 tells us the answer straight away. They give us 72 chairs. 9 x 8 = 72. So if we have 9 chairs on each row. 9 - 1 = 8

If the first answer can tell us what we need then the answer can only be A or D.

Number 2 tell us that if we subtract 1 chair from the last of two rows we have 17. If we add that chair back we have 18. 2 rows = 18. 18/2 = 9

Since the question does not ask to tell them how many rows there are in all then we can get the answer from Number 2 also. We are simply trying to find out "How many chairs are in a row".

Hope this helps.

Dont rush this one:

for A you need to solve the equation r(r+1) = 72 and since this can have two solutions for r. It's good time spent on finding values of r=8 and r=-9. since rows can not be -ve so SUFF

for B the eq is 2x chairs-1 = 17 so chairs in a row = 9 SUFF

hence D