Machine A, working alone at its constant rate, produces x pounds of peanut butter in 12 minutes. Machine B, working alone at its constant rate, produces x pounds of peanut butter in 18 minutes. How many minutes will it take machines A and B, working simultaneously at their respective constant rates, to produce x pounds of peanut butter?

Tow trains running at the rate of 75km and 60km an hour respectively on parallel rails in opposites directions, are to pass each other in 8 seconds and when they are running in the same direction at the same rates as before, a person sitting in the faster train observes that he passes the other in 33$\frac{1}{2}$ seconds. Find the lengths of the trains.

pot contains 81 litres of pure milk $\frac{1}{3}$ of liters of the milk is replaced by the same amount of water. Again $\frac{1}{3}$ of the mixture is replaced by that amount of water. Find the ratio of milk and water in the new mixture.

A train 400 meter long overtook a man walking along the line in the same direction at a speed of 5 km/hr and passed him in 40 seconds. The train reached the station in 20 minutes after it had passed the man. In what time did the man reach the station?

The speed of the boat in still water is 24 kmph and the speed of the stream is 4 km/hr. The time taken by the boat to travel from A to B downstream is 36 minutes less than the time taken by the same boat to travel from B to C upstream. If the distance between A and B is 4km more than the distance between B and C, what is the distance between A and B?