One approach, simple but with a caveat or two:
You need a double for "smallest absoulte ratio so far". Initialise it to something very big.
As you take in each line,
a) calculate the surface and the volume as two integers
b) calculate the absolute ratio as a double
c) if
the absolute ratio is bigger than the "smallest absolute ratio so far", ignore that line,
else
store your new "smallest absolute ratio so far" and the surface and volume integers
At end, reduce your two integers to their lowest terms by calculating out their prime factors and dividing by the ones they have in common.
Technically, there's a potential gotcha in using a double, since it's possible that two ratios are so close that a double doesn't have enough precision to distinguish between them. However, with dimensions only up to 50, I think you'll be safe.
Afterthought: the input could contain two identical lines, or lines that end up with identical answers, and if I were writing the test, I surely would put such identical lines in! That's OK, because the answer is the same.