How to make a random chance "function" in Batch

I'm making a small little game I want a random chance to get certain items (e.g. if the random number is greater than 10 but less than 15 then you will get a certain item). Here's what I've already tried which resulted in a crash.

    set /a chance= %random% %%30+1
if %chance% gtr 10 && lss 30 (
    set /a %nails%+1
    echo You got nails! %chance%
    )

This piece right here was just a test, but should give you an idea of what I am going for. This is really the only way I can think of doing it. If you could help, please do! :)