Answer:
He keeps 7/12 of the box of raisins
Explanation:
First of all you should know what fraction of the total he distributed among his brothers. For that, you must add
and
. You can see that both fractions have different denominators. So, to add, the first thing to do is put a common denominator. One of the easiest ways to get this denominator is just multiply the denominators of the two fractions:
6*4=24
Then the denominator in both fractions will be 24. But if you modified the denominator, the numerator. That is: if in fraction 1/6 you multiplied 6 by 4 to reach denominator 24, then numerator 1 must also be multiplied by 4. Then:

Similarly: if in the 1/4 fraction you multiplied the 4 by 6 to reach the denominator 20, then the numerator 1 must also be multiplied by 6. Then:

As now the 2 fractions have the same denominator, to add you leave the same denominator, which is 24, and add the numerators:

This fraction can be reduced, that is, express it in a simpler way. For that you divide the numerator and denominator by the same number. In this case, you can choose to divide the numerator and denominator by 2 and you get

So
is
. Then Chad gives his brothers
. To obtain the portion he keeps, 1 minus
is subtracted, 1 being the total. Taking into account that 1 =
and that the subtraction is done in a similar way to the sum previously explained, you get

So, he keeps 7/12 of the box.