Answer:
7.5 minutes
Step-by-step explanation:
The stacking rate working together is assumed to be the sum of the separate stacking rates. The rates are given in terms of minutes per pile, but we need to do the computation with rates in terms of piles per minute.
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total rate = Maria's rate + Alex's rate
= 1/20 piles/min + 1/12 piles/min = (12 +20)/(20·12) piles/min
= 32/240 piles/min = 2/15 piles/min
Then the rate in terms of minutes per pile is ...
1/(2/15 piles/min) = 15/2 min/pile = 7.5 min/pile
When both work together, it will take them 7.5 minutes to stack a pile of wood.
