In order to do this, you must make the denominators equal by finding their least common multiple. In this case, the LCM of 10 and 4 is 20.
Now whatever was used to multiply the denominator to make it equal twenty, you must also multiply the numerator by that number. So 10 was multiplied by 2 to make it 20 so 9 will also be multiplied by two, making the fraction 18/20. To make 4 become 20, it is multiplied by 5 so 1 will also be multiplied by 5. The new fraction is 5/20.
Now that we have like fractions, we can subtract (or add) them. 18 - 5 = 13. So the result is 13/20. To make a fraction it's simplest form, it has to be reduced to a point where it can no longer be reduced. We already know that 13 is a prime number so there is no way for it to be reduced. The answer is still 13/20.
