1. A and B working together can finish painting the house in 6 days. A, working alone can finish it in 5 days less than B. How long will it take each of them to finish the work alone?

2. A 400mm diameter pipe can fill the tank alone in 5 hours and another 600mm diameter pipe can fill the tank in 4 hours. A drain pipe 300 mm in diameter can empty the tank in 20 hours. With all the pipes open, how long will it take to fill the tank?

3. A merchant has 3 items on sale: namely, a radio for $50, a clock for $30, and a flashlight for $1. At the end of the day, she has sold a total of 100 of the 3 items and has taken exactly $1000 on the total sales. How many radios did he sell?

I don't have equations to actually get started. Help please!