Zen can finish a work in 20 days. He start doing the work and after 6 days John joined him. If both of them together completed the remaining work in 7 days, then in how many days can John alone finish 80% of the same work?

16 days

Let total work = 20

Zen can do it in 20 days, therefore,

Efficiency of Zen = 1 w/d

Work done by Zen in 6 days = 6

Remaining work = 20 - 6 = 14

Remaining work done by Zen + John in 7 days

Efficiency of Zen + John = $\frac{14}{7}$ = 2 w/d

Efficiency of John = 2 - 1 = 1

John will do 80% work in = $\frac{20\;×\;4}{5\;×\;1}$ = 16 days