If 3/7 of a pizza is eaten for lunch and 2/5 is eaten for dinner, hoe much of the pizza is eaten?

Ok so we need to start by getting both of these denominators into the same number. We must keep a constant ratio.

for this denominator, let use 35 because we know it works and its the least common multiple.

Since we multiply the bottom of the first fraction by 5, we must multiply the top the same way. This makes it 15/35.

The second one gets multiplied by 7, making it 14/35.

Let's now add up our fractions, getting us 29/35. We would simplify, but the answer is already in the most simple form.

Final 29/35 has been eaten, 6/35 is left over.