20.A firefighter spraying water on a fire stood on the middle of a ladder. The smoke lessened, so she moved up 3 rungs . It got too htot, so she backed sown 5 rungs. later, she went up 7 rungs and stayed until the fire was out. then, she climbed the remaiming 4 rungs and went into the building . How amny rungs does the ladder have/

Answer 1

from the middle to the top:

+3-5+7+4

double that