F(x)= 3x-27/14
Find f(f^-1(241))

Okay, the f^-1 means we need to invert the original function. So switch x's and y's and solve for y.
y = 3x - 27/14
x = 3y - 27/14
-3y = -x - 27/14
y =  1/3x + 9/14
Then plug in 241
y  = 80 1/3 + 9/14
y =  80 41/42

Now you have to plug it back into the original function.
So:
y = 3(80 41/42) - 27/14
y = 3401/42 - 27/14
y = 79.0476190476 or 79 and 1/21

To find the answer, you must divide it.

189 ÷ 21 = 9

You can buy 9 packs of batteries with your $189.