Since Loc can mow the lawn in 20 minutes, his rate is 1 lawn / 20 minutes or 1/20th of the lawn per minute.
Since Reza can mow the lawn in 30 minutes, his rate is 1 lawn / 30 minutes or 1/30th of the lawn per minute.
Loc mows for 5 minutes before he his joined by Reza. In these 5 minutes, he can mow (1/20)·(5) = 5/20th = 1/4th of the lawn.
Now, there is only 3/4th of the lawn to finish.
Assuming that the time they mow together is x, we can create this equation:
Amount done by Loc + Amount done by Reza = Total Amount
(1/20)(x) + (1/30)(x) = 3/4
Multiplying both sides by 60:
60(1/20)(x) + 60(1/30)(x) = 60(3/4)
3x + 2x = 45
5x = 45
x = 9 [It will take them 9 minutes, working together, to finish the lawn.]
Adding 9 to the 5 minutes that Loc mows alone = 14 minutes after Loc starts ---> 1:14 pm