Question 783511
With the old combine, Nikita's entire wheat crop can be harvested in 72 hours, but a new combine can do the same job in 48 hours.
 How many hours would it take to harvest the crop with both combines operating?
:
An easy way to do this:
let t = time when the two combines work together
Let the completed job = 1
Each combine does a fraction of the job, the two fractions add up to 1
{{{t/72}}} + {{{t/48}}} = 1
least common multiple of these two denominators, 144
144*{{{t/72}}} + 144*{{{t/48}}} = 144*1
Cancel the denominators, resulting in
2t + 3t = 144
5t = 144
t = 144/5
t = 28.8 hr working together 
:
:
Check this on your calc, enter
{{{28.8/72}}} + {{{28.8/48}}} =