Tom decided to go on a trip. During this trip, he needs to cross a lake, which in one direction takes 4 hours. During this time, Tom needs an assistant to help him with the crossing. Hiring an assistant costs $10 per hour. How much would Tom have to pay for help with crossing the lake back and forth?
Crossing the lake two times (back and forth) would take 4 * 2 = <<4*2=8>>8 hours.
So Tom would need to pay an assistant 8 * 10 = $<<8*10=80>>80.
#### 80