Medium 2

Many of them you can quickly solve, for example 91 is prime so must be square free, and 100 is itself a sqaure number, but if you want to be clinical about it you can break it down into prime factors, and if any factors are repeated in a number then it has a square factor. I've put the square factor (or the first if there are multiples) in brackets to point it out.

$90 = 2 \times (3 \times 3) \times 5$ $91 = 91$ $92 = (2 \times 2) \times 23$ $93 = 3 \times 31$ $94 = 2 \times 47$ $95 = 5 \times 19$ $96 = (2 \times 2) \times 2 \times 2 \times 2 \times 3$ $97 = 97$ $98 = 2 \times (7 \times 7)$ $99 = (3 \times 3) \times 11$ $100 = (2 \times 2) \times 5 \times 5$

That's 5 square-free.