Question 1198746
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The LCM of 30 and 20 is 60.


Let's say the task is moving 60 boxes.


Maria can move 60 boxes in 30 minutes.
Her rate is 60/30 = 2 boxes per minute.
rate = (amount done)/(time)
This is if Maria works alone.


Dana, when working alone, can do the same job in 20 minutes.
rate = (amount done)/(time)
rate = (60 boxes)/(20 min)
rate = 3 boxes per min


Given information: "One day Maria stated the task, then after 10 minutes she was joined by Dana".


If Maria works for 10 minutes, and her rate is 2 boxes per minute, then she has moved 2*10 = 20 boxes in that timespan.
amount done = (rate)*(time)


There are 60-20 = 40 boxes left to be moved.
The two women's individual rates add to 2+3 = 5 boxes per minute. 
This assumes they work efficiently without getting in each others' way.


If their task is to move 40 boxes, and their combined rate is 5 boxes per min, then:
(rate)*(time) = amount done
time = (amount done)/(rate)
time = (40 boxes)/(5 boxes per min)
time = <font color=red>8 minutes</font> is the final answer.
This is the amount of time they work together to finish up the task (moving the remaining 40 boxes).


The 60 I chose at the very start of the problem isn't special at all. 
You can replace it with any other positive integer you wanted and still get the same final answer. 
I chose 60 so that the divisions resulted in whole numbers.
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