Joel made muffins, he gave 1/4 to his neighbor and 3/8 to his school how may muffins were left

When you're adding fractions with two different denominators, the best thing to do is to find a common denominator;
In this case, 4 and 8 are both multiples of 8, and therefore, you've got to double the first fraction.
Doubling the first fraction gives you 2/8 and 3/8. You've then got to add these together, and this gives you 5/8 (as you only add the numerators)
As you're looking for what is left, you have to do 8/8- 5/8 (once again, only subtracting the numerators)
8/8- 5/8= 3/8
Therefore, 3/8 of the muffins are left.
Hope this helps :) 

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