A takes 20 days---> 1 day work done by A is 1/20
B takes 30 days---> 1 day work done by B is 1/30

Together in 1 day---> 1/20=1/30=5/60=1/12 work is done

A quit 10 days before the project was completed which means B worked alone for 10 days and therefore complete 10*1/30 = 1/3rd of the total work.

2/3rd of the work was done by A and B together at 1/12th work per day.
Therefore days taken to complete work together = (2/3)/(1/12) = 24/3= 8 days

Therefore total days taken for completion of the project is 8+10 = 18 days

dmateer & parallel_chase,

I think I did make the assumption that A quits 10 days before the time A&B together would've completed the job.
Going with the nature of the GMAT, this q could'nt have been that straightforward

Anyway thanks for pointing out the "fatal" consequences of assuming wrong on the GMAT.