First, you need to find how long 1 man takes to build a trench 6m deep. So you multiply the time by 6 because when you have lesser men, you will require more time to build for the same depth. Therefore, 1 man takes 6*4 = 24 hours to build a trench 6m deep. Now we can find how long 10 men take to build a trench 6m deep, by dividing the time. 10men will take 2.4 hours to build a trench 6m deep. Now we will keep the men constant, and find how long it takes for 10 men to build a trench 1m deep. As the depth is lesser, the time taken to build will also be lesser, therefore we divide the time by 6. So, 10 men will take 2.4/6 = 0.4 hours to build a trench 1m deep. Now we find the time taken for 10 men to build a trench 5m deep by multiplying the time by 5. Therefore the time taken for 10 men to build a trench 5m deep is 0.4*5 = 2 hours.

Wanted to add that u need to observe which variables have an inverse/ direct relationship can’t just anyhow times