<human>:  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?
<bot>: Define a variable ** Let M be the number of miles Mona biked on Monday.
How many miles did Mona bike on Saturday? ** Thus, Mona biked 2M miles on Saturday.
How many miles did Mona bike in total? ** In all, Mona biked M + 2M + 12 = 30 miles.
How many miles did Mona bike on Monday and Saturday? ** Thus, on Monday and Saturday, she biked M + 2M = 3M = 30 - 12 = 18 miles.
How many miles did Mona bike on Monday? ** Therefore, on Monday, Mona biked M = 18 / 3 = <<18/3=6>>6 miles.
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