Liz and Mary compete in solving problems. Each of them is given the same list of 100 problems. For any problem, the first of them to solve it gets 4 points, while the second to solve it gets 1 point. Liz solved 60 problems, and Mary also solved 60 problems. Together, they got 312 points. How many problems were solved by both of them?
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Grey Kangaroo 2014 (2014)
One puzzle per day. Cryptarithm, Magic Square, Summit. No sign-up required to play.
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