That is not correct.
This is what you did wrong:
You divided 500,000 dollars by 100,000,000 calls and got 0.005 ___ per ___.
Your mistake was assuming that because you saw a fractional result that you were dealing with cents. That is not correct. Your unit is still dollars, so your result should be 0.005 _dollars_ per _call_.
That's 5/1000 of a dollar per call. To find cents, you'll need to multiply that result by 100. (Remember: dollars = cents/100; cents = dollars*100) That will give you the correct answer of 0.5 cents per call.
You can check this yourself by first converting dollars to cents, then doing the division: 500,000 dollars = 50,000,000 cents. 50,000,000 cents / 100,000,000 calls = 0.5 cents per call.
If you're still struggling, try working the problem from a different angle. First, find the number of calls per dollar: 100,000,000 calls / 500,000 dollars = 200 calls per dollar. Divide by 100 to find the number of calls per cent and you'll find that you can make 2 calls per cent. 1 cent / 2 calls = 0.5 cents per call.
Check your work by working the problem backwards. At 0.5 cents per call, how much would it cost to make 100,000,000 calls? 100,000,000 calls * 0.5 cents = 50,000,000 cents. 50,000,000 cents / 100 = 500,000 dollars.
There will be a quiz on Monday, so be sure to study over the weekend.