Question 664324
Rate(Steven): 20 / 5 min
Rate(Sue): 20 / 10 min
Rate(Jack): 20 / 15 min
Combined rate:
20/5 + 20/10 +  20/15 = 20/x where x is their combined rate
Divide both sides by 20
1/5 + 1/10 + 1/15 = 1/x
Create common denominators
6/30 + 3/30 + 2/30 = 1/x
11/30 = 1/x
Knowing 1/x is the inverse of x, invert the other side as well
x = 30/11 = 2 8 /11 = 2.72 minutes
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