This is a bit of a gift, but take it and use a similar logic, to do the second part of your assignment.
int start = 10;
int end = 20;
const double litersPerGallon = 3.785; //edit: not constant, const
double liters = 0.0;
printf("gallons to liters, from 10 to 20:\n");
while ( start <= end )
liters = start * litersPerGallon;
printf("\n %d gallons is %4.3lf liters", start, liters);
You need to "marinate" in this kind of logic, because it's quite basic, and used all the time. Let it soak in, very well.