Two cross-country running teams participated in a (hypothetical) study in which a fraction of each team used weight training to supplement a running workout. The remaining runners did not use weight training. At the end of the season, the mean improvement in race times (in seconds) was recorded in the table below.

Describe how Simpson's paradox arises in this table. Resolve the paradox by finding the percentage of each team that used weight training.

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Mean improvement (seconds) WeightNo weightTeam training training average Gazelles 10 6.0 Cheetahs 6.2