First taking into account the if he buys in order.
Brian has 10 choices for the first selection of fish, 9 for the second selection and 8 for the third selection.
Thus you have 10*9*8 = 720 possible ways
However the question did not specify if it is in order. Now counting how many possible orders for each selection you can have
fish 1 , fish 2 , fish 3 or fish 1 , fish 3 , fish 2 or fish 2 , fish 1 , fish 3 or fish 2 , fish 3 , fish 1 or fish 3 , fish 1 , fish 2 or fish 3 , fish 2 , fish 1 or
We have 6 order.
dividing the possible ways by the order we have 720 / 60 = 120
