HTML problem

1) Drivers are concerned with the mileage obtained by their automobiles. One driver has kept track of several tankfuls of gasoline by recording the number of miles driven and the number of gallons used for each tankful. The script should calculate and output HTML5 text that displays the number of miles per gallon obtained for each tankful and prints the combined number of miles per gallon obtained for all tankful up to this point. Use prompt dialogs to obtain the data from the user. Have a suggestion for the user to insert "-1" to exit the prompt and show the calculated result.