Evaluate each of the following values for the f(×)=[×]:

We know that C is 385, so we can plug that in to get the equation 385=15n+85. To find the number of watches, we have to solve for n.
To do that we have to...

385=15n+85

First subtract both sides of the equal sign by 85 (because we need to do the opposite)

300=15n (because 85-85 is zero, so we eliminated that, and 385-85 is 300)

Now we divide both sides of the equal sign by 15 (again, opposites)

20=n (because 15n/15 leaves us with n since both 15s cancel out, and 300/15 is 20)

So the number of watches made were 20.