1. If Sally can paint a house in 4 hours, and John can paint the same house in 6 hour, how long will it take for both of them to paint the house together?

2. The sales price of a car is $12,590, which is 20% off the original price. What is the original price?

3. If Leah is 6 years older than Sue, and John is 5 years older than Leah, and the total of their ages is 41. Then how old is Sue?

4. Alfred wants to invest $4,000 at 6% simple interest rate for 5 years. How much interest will he receive?

I'm not asking you to do the work for me. I'm just confused as to how to find the right formula to solve the equation. Each equation I come up with seems to be wrong and I get the wrong answer. I just need a little guidance. Thank you in advance.