Work done by 'A' in 15 days = 1 work
Work done by 'A' in 1 day = 1/15 part of a work
Work done by 'B' in 30 days = 1 work
Work done by 'B' in 1 day = 1/30 part of a work
Wwork done by (A+B) in 1 day = 1/15+1/30 = 3/30 = 1/10 part of a work.
Hence the (A+B) will complete the work in 10 days.