What formula is used to determine If sally cfan paint a house in four hours and john can paint the same house in 6 hours how long will it take for both of them to paint the house together?

Answer 1

You don't need a formula. You need to understand where the answer comes from.

If you understand where the answer comes from, you can write your own formula.

If you don't, then a formula can be dangerous in your hands.

-- Sally can paint 1/4 of the house in an hour.

-- John can paint 1/6 of the same house in an hour.

-- If they work together, then can paint (1/4 + 1/6) = 5/12 of the house in an hour.

-- If they paint 5/12 of the house in an hour, then it takes them 12/5 hours to

finish the job, or 2.4 hours, or 2hours 24minutes.

Well, OK. Here's the formula:

S = number of hours it takes Sally

J = number of hours it takes John

Number of hours it takes them working together = 1 / (1/S + 1/J)