Grace can read 200/20 = <<200/20=10>>10 pages in 1 hour.

So she can finish a 250-page book in 250/10 = <<250/10=25>>25 hours.

# Answer

25
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Grace can read a 200-page book in 20 hours. How long would it take her to finish reading a 250-page book at this constant rate?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
25
Grace can read a 200-page book in 20 hours. How long would it take her to finish reading a 250-page book at this constant rate?
train