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You'll need to add some new variables to store the running totals for hours and charge. Then, as you loop through your vehicles, you need to update those running totals for each vehicle in turn.
By the way, your code will be much simpler if you simply use an array to hold the values entered by the user. You won't need to do all that messy business of setting x to a, b, or c depending on the value of count. You'll be able to simply access each array element using the loop index.