Problem 1:

<still working on it>

Problem 2:

Option 1: Fill the three liter jug three times, emptying it each time into the tub.

Option 2: Fill the 5 liter jug and pour it into the 3 liter job, allowing the remaining two liters to overflow into the tub. Do this again, so that you have a total of 4 liters in the tub. Finally, fill the 5 liter container and poor the whole thing into the tub.

Problem 3:

7*(3+3/7) = 24