Question 866734
The rates of each crew are:

{{{1/12}}} and {{{1/10}}} JOBS per HOUR.


The way uniform rates situations go, {{{R*x=y}}}.  In the kind of application for your example, x is time in hours and y is how much or many jobs.  The unit of R is jobs per hour and R is a rate.  The rate of each crew working together is the sum of their individual rates, so:


{{{1/12+1/10=10/120+12/120=(10+12)/120=22/120=highlight(11/60)}}} jobs per hour.


Now, if rain will come in 5 hours, will this 5 hours be enough time to do ONE complete job?

{{{highlight_green((11/60)*5>=1)}}}  ?   Is this true or false?


{{{55/60>=1}}} FALSE.
The two crews together will not be able to finish the roof before the rain prediction time.