Hi!
To subtract fractions, you have to operate like when you are adding fractions. That is to say, start by putting everything on the same common denominator (the denominator is the number at the bottom of the fraction;)). In this case, you have to find the common denominator of the 2 fractions, which is the number 30.
For the first fraction (5/10), to go from 10 to 30, we have to multiply by 3 at the bottom, then by 3 at the top. The fraction therefore becomes 15/30.
For the second fraction (3/15), to go from 15 to 30, we have to multiply by 2 at the bottom, then by 2 at the top. The fraction becomes 6/30.
The sole thing remaining is to subtract: 15/30 - 6/30 = 9/30
Finally, we must simplify the fraction. To do this, divide the upper part (numerator) and downer part of the fraction (denominator) by the same number to arrive at an irreducible fraction. As the name implies, an irreducible fraction is a fraction which cannot be reduced further lower (only by 1).
In this case, let's divide by 3 at the top and by 3 at the bottom, which gives us 3/10! The final answer is therefore 3/10
Don't hesitate if you have any other questions!