For this one: 28 - 9
If the problem was 28 - 8, would you agree with me that the answer would be 20? So now if I take one more away, that is 28 - 9, I should have 19 right?
When simplifying ones like 21/35, see if you can see a common factor in the numerator and denominator. I see a 7 (maybe there are more, but we can deal with them one at a time), that is,
Now, you should be able to convince yourself that 7/7 = 1. So our fraction becomes
So your second answer looks good. As an additional note, since 3 and 5 are both prime numbers, we can instantly say this can not be simplified any further.
Hope that helps.