#1:

Both workers can paint 1/6th of the house in one day.

Therefore,

Solve for t.

#2:

The 400mm pipe can fill 1/5th of the tank in one hour and the 600mm can fill 1/4th of it in one hour. But another empties 1/20th of it in one hour.

Solve for t.

For #3, did you forget a portion of the statement?. I may be mistaken, but it appears there's not enough info. The way it is you have two equations and 3 unknowns.