MathRevolution wrote:

There is a total of 1,800 pages that needs to be typed. If it takes 90 minutes when a types alone, 45 minutes when b types alone, and 30 minutes when c types alone. How many minutes does it take if 3 of them work together?

A. 10min

B. 12min

C. 15min

D. 18min

E. 20min

Another way: calculate rates using 1800 as \(Work\), then divide by time \(t\) = number of minutes it takes each person to complete all 1800 pages.

\(r= W/t\)

a = 1800 p/90 mins is \((\frac{20p}{1 min})\)

b = 1800 p/45 min is \((\frac{40p}{1 min})\)

c = 1800 p/30 min is \((\frac{60p}{1 min})\)

In one minute, the three together can type 20 + 40 + 60 = 120 pages, i.e., \((\frac{120p}{1 min})\)

1800 pages/120 pages in 1 minute = 15 minutes to finish when working together

Answer C

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