Alex can paint a wall in 4 hrs. Beth can paint the same wall in 5 hrs. How much time will it take to paint the wall if they work together?

To solve this problem, we need to find out how much of the wall each person can paint per hour, then add these amounts together to find out how much they can paint per hour when working together.

1. Alex can paint a wall in 4 hours, so Alex can paint 1/4 of the wall in one hour.
2. Beth can paint a wall in 5 hours, so Beth can paint 1/5 of the wall in one hour.

If they work together, they can paint (1/4 + 1/5) of the wall in one hour.

To add these fractions, we need a common denominator. The least common denominator of 4 and 5 is 20.

So, we convert 1/4 to 5/20 and 1/5 to 4/20.

Adding these together, we get (5/20 + 4/20) = 9/20.

This means that together, Alex and Beth can paint 9/20 of the wall in one hour.

To find out how long it would take them to paint the whole wall, we divide 1 (the whole wall) by 9/20, which gives us approximately 2.22 hours.

So, it would take Alex and Beth approximately 2.22 hours to paint the wall if they work together.