For 1 complete job, it takes 9 hour for Isabella to finish it alone.
So, her rate is:
`Rate=(Work d o n e)/(time)`
Since Matthew works three times as fast as compared to Isabella, then his rate is:
If they work together to do 1 complete job, their total rate is:
Since total rate is also equal to work done ( which is assumed as 1 complete job) divide by total time `(R_T=1/T)` ,then the equation is:
To simplify the equation, multiply both sides by the LCD of the two fractions which is 9T` `.
Then, divide both sides by 4 to isolate T.
Hence, working together, it will take them 2.25 hours to rake the leaves.