Question 1032872
The exercise is intended as a uniform rates problem, each "worker" being a heater, working at its own rate.  You can make a chart or table of rates.


One job is "warm the room air from 40 to 70 degree".


<pre>HEATER          RATE

A               1/5

B               1/7</pre>


Another chart may be what you want, but the rest of the solution will be through equations only.  The rule to use will be  RT=J relating rate, time, job.


{{{highlight_green((1/5)(2.5)+(1/7)x=1)}}} for the heaters working separately, to do ONE JOB.  The unknown time for heater B to work to finish the job is x.


Once that is understood, solving for unknown time x is uncomplicated arithmetic.