The number of minutes required to wash 12 windows is 8 4/7 minutes.
According to the given question.
You can wash one window in 15 minutes, so each minute you can wash 1/15.
Your sister can wash one window in 20 minutes, so each minute the sister can wash 1/20.
If working together you can wash 1/15 + 1/20 = 7/60.
Hence, the time required to wash 12 windows together
= 12 × 7/60
= 8 4/7 minutes.
Therefore, the number of minutes required to wash 12 windows is 8 4/7 minutes.
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