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This question requires that you find the Lowest Common Multiple (LCM) of the 2 girls time:
4min = 2 x 2
6 min = 2 x3
LCM = 2 x 2 x 3 = 12 min
Every 12 minutes, Diana will make 3 cranes + Jane will make 2, so 5 cranes/12min.
To make 30 cranes, they would thus need 6 x 12 min = 72 min.
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