If Keith pays $70, we know that $14 would have been paid, regardless of how long he rented a bike for. 70 - 14 = 56, so $56 would have been paid depending on the time he rented the bike for. If the cost per hour is $7, then the number of hours Keith rented the bike for is just $56 ÷ $7 per hour, which equals 8 hours.
Keith rented the bike for 8 hours.
Hope this helps!