Question 1142603
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The other tutor has provided a good response showing the traditional algebraic method for solving this kind of "working together" problem.<br>
The problem as posed appears to require an algebraic solution.  However, when a problem like this does not require an algebraic solution, here is an alternative method you can use that I find many students like better.<br>
(1) Consider a common multiple of the two given times.<br>
(2) The LCM of 7.5 and 20 is 60.  7.5*8 = 60; 20*3 = 60.<br>
(3) In 60 hours, the one student alone can build 3 of the computers; in 60 hours the two of them together can build 8 of them.<br>
(4) So the other student can build 5 of the computers in 60 hours; that means he can build 1 computer in 60/5 = 12 hours.