(A + B)'s 1 day's work = $1/30$ 
(A + B)'s 20 day's work = $20/30 = 2/3$
Remaining work to be done by A = $1 - {2/3} = 1/3$
$1/3$ work is done by A in = $20/1 × 3$ = 60 days.
or A's 1 day's work = $1/60$ 
B's 1 day's work = $1/30 - 1/60 = 1/60$
Hence, B can finish the work in 60 days.