Answer:
4 hours = 21 workers 
Explanation:
Given information,
→ About 24 workers is needed if the work duration is 3.5 hours. 
Now we have to,
→ Find the workers needed if 4 hours. 
Let's assume that,
→ Needed workers = N
Forming the equation,
→ 24 × 3.5 = N × 4 
Then the value of N will be,
→ 24 × 3.5 = N × 4 
→ N × 4 = 24 × 3.5 
→ N × 4 = 84 
→ N = 84/4 
→ [ N = 21 ] 
Therefore, the answer is 21.