PLEASE HELP ASAP

A special piece of equipment is used at a construction site to dig holes. A hole is dug to a depth of 3.25 feet when the equipment hits rock. The progress slows to 5/8 feet per hour of digging. The depth of the hole must be at least 11 1/2 feet by the end of the day to stay on schedule.

What is the minimum amount of time the equipment must continue at its present rate to complete the hole on schedule?