Have you specified the question completely ? My first thought was to wonder " A percentage of WHAT ?" If you want 31 as a percentage of another number, then you multiply 31 by 100 and divide by the other number.
If you have found an answer which is 31 times the original quantity, then indeed 31 is 3100% of the original ( but notice that this is an INCREASE of only 3000% - that is, (31 - 1) x 100 )