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.

# Answer

80
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
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?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
80
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?
train