Answer: (4) 13.33 days

Solution

let’s say work is 20 units

rate of A = 20/20 = 1 unit a day

now A does 4 units of work in 4 days. 

Remaining work = 16 units, b does in 8 days, hence rate of b = 2 units/day

now combined rate of A and B = 1+2 =  3 units/day

so number of days when they work together = 20/3 = 6.66 days

So twice the work will be don is 13.33 days