In an hour Riyad can finish 1/2 of the assignment and Omar can finish 1/3 of it. Let x be the amount of time it will take Omar to finish the work. The equation that would best relate the conditions above is,
(1/2 + 1/3)(1) + (1/3)(x) = 1
The value of x from the equation is 0.5 hours. Therefore, it will take Omar another 30 minutes to finish the work.