Amy and Bob decide to paint one wall of a building. Working alone, Amy takes 12 hours to paint the entire wall while Bob takes 18 hours for the same. Amy painted the wall for 4 hours and then Bob took over and completed the wall. How long did it take for them to paint the entire wall

16 hours

Step-by-step explanation:

From the above question, we are given the following information

For one wall, working alone,

Amy can paint for 12 hours

Which means, in

1 hour , Amy would have painted = 1/12 of the wall

Bob can paint for 18 hours

Which means ,

in 1 hour, Bob would have painted = 1/18 of the wall.

We are told Amy painted the wall for 4 hours and then Bob took over and completed the wall.

Step 1

Find the portion of the wall Amy painted before Bob took over.

Amy painted the wall for 4 hours before Bob took over.

If:

1 hour = 1/12 of the wall for Amy

4 hours =

Cross multiply

4 × 1/12 ÷ 1

= 4/12 = 1/3

Amy painted one third(1/3) of the wall

Step 2

Find the number of hours left that Bob used in painting the remaining part of the wall

Let the entire wall = 1

If Amy painted 1/3 of the wall

Bob took over and painted = 1 - 1/3

= 2/3 of the wall

If,

Bob painted 1/18 of the wall = 1 hour

2/3 of the wall = ?? = Y

Cross multiply

2/3 × 1 = 1/18 × Y

Y = 2/3 ÷ 1/18

Y = 2/3 × 18/1

Y = 36/3

Y = 12 hours.

This means, the number of hours Bob worked when he took over from Amy = 12 hours.

Step 3

The third and final step is to calculate how many hours it took them to paint the wall

Number of hours painted by Amy + Number of hours painted by Bob

= 4 hours + 12 hours

= 16 hours

Therefore, it took them 16 hours to paint the entire wall.