The relationship of inversely proportional variables is:

In this case y is the amount of grant and x the number of people. You found k (and it's correct) so simply sub your value of k in when x = 120

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Another way to think about it is that every time the number of peopledoublethe grant will behalfof the previous amount.

As 120/30 = 4 then it follows it will be a quarter of the original amount: 60/4 = 15