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? Think carefully first, then make a decision:
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 = 6 miles. So, the answer is 6.