To answer this question, you'd need to know the approx. thickness of a nickel. Was this info not given to you in the problem statement?
Let's assume that the thickness of a nickel is 1.5 mm. What fraction of an inch is that?
1.5 mm 1 inch 1 cm
----------- * ------------ * ------------ = 0.06 inch
1 2.54 cm 10 mm
How many nickels would it require to make a stack 100 inches tall?
Divide:
100 inches
--------------------------- = 1667 nickets (approximately)
0.06 inch / 1 nickel
Note that this response is based upon my estimate that a nicket is 0.06 inch thick. If you want a more accurate answer, you'll need to measure the thickness of a nickel or look it up somewhere.