Sanjay and Tannu, working separately, can do a piece of work in 6 and 20 days, respectively. If they work for an alternate day, beginning with Tannu, in how many days will the work be completed?

Correct option is A

Given:
Sanjay and Tannu, working separately, can do a piece of work in 6 and 20 days, respectively. If they work for an alternate day, beginning with Tannu.
Solution:​
Let the total work = LCM of 6 and 20 = 60 unit
Efficiency of Sanjay = $$\frac{60}{6}= 10$$ unit per day​

Efficiency of Tannu = $$\frac{60}{20}$$​ = 3 unit per day
In 2 days = 13 unit of work completed by Sanjay and Tannu.
In 8 days = 52 unit of work will be completed
Then, 
9th day = 3 unit of work will be completed by Tannu
Then, Remaining work = 60 - 52-3 = 5 unit of work will be completed by Sanjay.
Time taken to complete the reamining work = $$\frac{5}{10} = 0.5 day$$ 
So, total time taken to complete the whole work = 9 + 0.5 = 9.5 days