The average formula is \(\frac{S}{N}= Average\) OR \(S = Average * N\)

Now we do have that your first average is 160, from this follow that \(S = 160N\)

If you add a number I.E n + 1 that is 170 you end up with : \(\frac{S + 170}{n + 1} = 161\) (the new average)

So S = 160N from this we do have \(\frac{160N + 170}{N +1} = 161(N + 1 )\)

Solving you do have \(N = 9\)

Hope this helps

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