There is an example hope it helped
2) Cereal cost $7 / box. How many boxes did Huong buy if she spent $21?
2 answers:
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A perfect square must be hidden within all of those radicands in order to simplify them down to what the answer is. 
. 
. 
. The rules for adding radicals is that the index has to be the same (all of our indexes are 2 since we have square roots), and the radicands have to be the same. In other words, we cannot add the square root of 4 to the square root of 5. They either both have to be 4 or they both have to be 5. So here's what we have thus far: 
. We can add 
and 
to get 
. That means as far as our answer goes, A = 72 and B = 4, or (72, 4), choice a.