a firefighter spraying water on a fire stood on the middle rung of a ladder. when the smoke became less think, the firefighter moved up 4 rungs. however it got too hot, so the firefighter backed down 6 rungs. later, the firefighter went up 7 rungs and stayed until the fire was out. then , the firefighter climbed the remaining 4 rungs and entered the building. how many rungs does the ladder have?