A truck needs 7 gallons of fuel to travel 56 miles. Can the truck travel 48 miles with 6 gallons of fuel

56 divided by 7 = 8

48 divided by 6 = 8

8 = 8

since it is the same rate, then

Yes, the truck can travel 48 miles with 6 gallons of fuel

All thing held equal, yes 6 gallons should work. The truck can travel 8 miles per gallon, or 7 gallons times 8 equals 56 miles. Now if there were variables such as traffic or hills or turning on the air conditioning, that could affect how many miles per gallon the truck gets.