Frans fudge recipe calls for 2 1/2 cups of sugar and she increases it by 2/5 of a cup, how much sugar does she use?

For this you are adding two fractions together. You always want to make sure they are both improper or normal fractions and that they have a common denominator. To find a common denominator, you look at the denominators and look for a number (The smaller the better) that both can be multiplied by some number to get to. In this case, the common denominator is 10. To make the denominators the same you multiply by one so the value doesn't change. This is done by multiplying the numerator and denominator by the same number that results in the denominator becoming the common denominator.
2 1/2 = 5/2 × 5/5 = 25/10
2/5 × 2/2 = 4/10

When adding, the denominator remains the same and the numerators are added!
25 + 4 = 29
So the answer is 29/10 or 2 9/10 when converted back into a mixed number.
Hope this helped!