SOLUTION: if 3 people can build a wall in 25 minutes, how long will it take for 5 people to build the same wall

First way:

The LCM of 3 people and 5 people is 15 people.  15 people can build a wall in
1/5 the the time that only 3 people can or 1/5th of 25 minutes, or 5 minutes.
5 people will take 3 times as long as 15 people, and 3 times 5 minutes is 15
minutes.
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Second way:

Use the worker-time-job formula, which is:



where

W1 = the number of workers in the first situation.
T1 = the number of time units (minutes in this case) in the first situation.
J1 = the number of jobs in the first situation.

W2 = the number of workers in the second situation.
T2 = the number of time units (minutes in this case) in the second situation.
J2 = the number of jobs in the second situation.

W1 = 15             W2 = 5     
T1 = 25             T2 = the unknown quantity 
J1 =  1             J2 = 1











Answer: 15 minutes.
  
Edwin