Four men finish painting a wall in 10 days. Six boys do the same job in 8…

General principle: each worker's individual daily work rate is 1 / (number of such workers x days that group alone takes); daily rates of different worker types add together to give the combined daily rate; the time to finish a given amount of work equals that amount divided by the combined daily rate.

1. Rate of 1 man = 1/(4x10) = 1/40 of a wall per day.

2. Rate of 1 boy = 1/(6x8) = 1/48 of a wall per day.

3. Rate of 8 men = 8x(1/40) = 1/5 of a wall per day.

4. Rate of 8 boys = 8x(1/48) = 1/6 of a wall per day.

5. Combined rate = 1/5 + 1/6 = 6/30 + 5/30 = 11/30 of a wall per day.

6. Time for 1 wall = 30/11 days.

7. Time for 2 walls = 2 x (30/11) = 60/11 days.

Cross-check: (60/11) days x (11/30) wall per day = 2 walls, matching the requirement of two walls -- confirming the result.

So 8 men and 8 boys working together will paint two such walls in 60/11 days.