Answer: 10 Hrs

Amount of water drained by three taps = 1/5 = 0.2 i.e 20%

Amount of water drained by 1st tap = 1/10 = 0.1 i.e 10%

let x and y be time taken by 2nd and 3rd tap respectively,

==> 1/x + 1/y + 0.1 = 0.2

1/x + 1/y = 0.1

if we assume 3rd tap drains neglegible amount of water,

then, 1/x = 0.1 ==> x = 10 hrs