15/1-20/3 can also be written 15-20/3.
We need to get numbers over one common denominator, so that denominator is 3 (or 3*1).
Take 15/1 and divide 1 into 3. It goes 3 times so we multiply 3 by 15=45 and subtract 20 from it=25.
So now we have 25/3. That's the answer but by dividing 3 into 25 we get 8 remainder 1, which is 1/3, and the alternative answer is 8 1/3.