The charge should be $6. I believe I set this up correctly; let me know if there are any errors. Hope this is helpful & accurate.
What are 3 ratios that are equivalent to 18:4
2 answers:
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One way you could solve this is to just multiply the top and bottom out so that you get 9/81, reducing it by 9/9 to get 1/9 or option C.
Another way would be to do
since dividing numbers with exponents would be subtracting the bottom exponent from the top exponent, provided that the base number (in this case 3) is the same for both. For this method, you would get 
, which is equal to 1/9 or .1 repeating, the same answer that you'd get with the first method.
Another way would be to do