simple math problem

Question

Kelsi can type a report in 3 hours and Ashley takes 4 hours. How long will it take the two of them typing together?

Answer 1

This is a rate problem.  The key idea is that rates add.  

Kelsi's rate is 1report/3 hrs ,   Ashley's ratie is 1report/4 hrs.  So the combined rate is 1/3 + 1/4 = 7/12 report/hr. 

The time required, T, when multiplied by the rate (7/12) will equal one report.    So

 

(7/12) T = 1   and   T =  12/7 hrs.