Answer: approximately 26.66667 minutes
This converts to 26 minutes, 40 seconds
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Step-by-step explanation:
Let's say there are 360 newspapers that need to be delivered.
The two brothers can get the job done in 40 minutes if they work together. Their rate is 360/40 = 9 newspapers per minute.
Richie can do the full job alone in 90 minutes. So his rate would be 360/90 = 4 newspapers per minute. In 30 minutes, he delivers 30*4 = 120 newspapers. This leaves 360 - 120 = 240 left over.
Divide this over the rate of 9 papers per minute to get 240/9 = 26.66667 approximately. It takes about 26.66667 minutes to finish up delivering the 240 remaining newspapers if the brothers work together.
The portion 0.66667 multiplies with 60 to get 0.66667*60 = 40.0002 which rounds to 40 (we have rounding error going on). This means
26.66667 minutes = 26 minutes, 40 seconds