Question 389193
Let R1, R2, and R3 equal the rates in tankfuls/hr
that the pipes fill the tank
(1) {{{R[1] + R[2] + R[3] = 1/6}}}
(2) {{{R[1] + R[2] = 1/6}}}
Subtracting (2) from (1), I see that {{{R[3] = 0}}},
So P3 contributes nothing to filling the tank,
so it would never fill the tank. Are you sure you 
copied this problem correctly?