P works twice as fast as Q. If Q can complete a task in 36 days independently, the number of days in which P and Q can together complete the task is:

Given that P works twice as fast as Q, the ratio of their efficiencies is:
Efficiency of P:Efficiency of Q=2:1
Since efficiency is inversely proportional to time taken:

Time taken by P=
2
Time taken by Q
​
=
2
36
​
=18 days
Work done by P in 1 day =
18
1
​

Work done by Q in 1 day =
36
1
​


Together, work done by P and Q in 1 day is:

18
1
​
+
36
1
​
=
36
2+1
​
=
36
3
​
=
12
1
​

Thus, working together they will complete the task in 12 days.