Let M be the number of miles Mona biked on Monday.

Thus, Mona biked 2M miles on Saturday.

In all, Mona biked M + 2M + 12 = 30 miles.

Thus, on Monday and Saturday, she biked M + 2M = 3M = 30 - 12 = 18 miles.

Therefore, on Monday, Mona biked M = 18 / 3 = <<18/3=6>>6 miles.

# Answer

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<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Mona bikes 30 miles each week to stay fit. This week, she biked on Monday, Wednesday, and Saturday. On Wednesday, she biked 12 miles. On Saturday, she biked twice as far as on Monday. How many miles did she bike on Monday?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
6
Mona bikes 30 miles each week to stay fit. This week, she biked on Monday, Wednesday, and Saturday. On Wednesday, she biked 12 miles. On Saturday, she biked twice as far as on Monday. How many miles did she bike on Monday?
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