Imagine cutting up a pizza into 5 slices
If you take 2 slices, then you take 2 out of the 5, so the fractional amount you took is 2/5 of the full pizza.
If your friend takes 3 slices, then she or he takes 3/5 of the full pizza
Adding the two fractional amounts should get us back to 1 full pizza, which is why 2/5+3/5 = 1.
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We could say this:
2/5 + 3/5 = (2+3)/5 = 5/5 = 1
Dividing any number over itself, as long it's not zero, always leads to 1.
So that's why 5/5 = 1.