One way to look at its is filling in different regions. If you have an object n and need to fill its first region you have n possibilities you have n¹ combinations. Then for the second region you would have n possibilities for each region, so nn=n^2. Then for the third region, nn*n=n^3. So im sure you see the pattern that for an object n, with k regions you have n^k ways to fill that region.

Using the example above in the water molecule there is only one oxygen region and there are only 16 isotopes to choose from so 16^1. However there are two hydrogen regions each can be filled by one of 7 different isotopes thus there are 77 or 7² combinations. Thus in total there are 167² = 784 combinations.

Sorry about the formatting im on mobile.