Hi,

It takes Nina 3 hours to proof a chapter of Hawkes Learning Systems' Statistics

 book and it takes Cynthia 12 hours.

Let x represent the time it would take them together

Note: job / hour is the key

 1/3 + 1/12 = 1/x       |Multiplying thru by 12x so as all denominators = 1

4x + x = 12

   5x = 12

    x = 2.4 hrs, the time it would take them together 

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