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Hint: At first find out the value of a unit. That means how many workers will be required to do the work in one hour. Then calculate how many workers will be required to do the work in 35 hours.

It is given in the question that 7 workers can build a trench in 25 hours. First we have to find out how many workers will be required to build the same trench in one hour.

We can solve this problem using a unitary method in which we find the value of a unit and then the value of a required number of units.

Always write the things to be found on the right hand side and things known on the left hand side.

Here we have to find out how many workers we need. So, the unknown is the number of workers. We have to put this on the right hand side.

We know about the hours. So, we will take hours on our left hand side.

In 25 hours 7 workers can build a trench.

If we have to finish the same work in one hour we will need more manpower. Therefore,

In 1 hour $7\times 25$ workers can build the trench.

In 35 hours $\dfrac{7\times 25}{35}$ workers can build the trench.

So, we need $=\dfrac{7\times 25}{35}=5$ workers to build the same trench in 35 hours.

Hence, we need 5 workers to build the same trench in 35 hours.

Note: We generally make mistakes when we calculate the value of a unit. We need to understand by reading the question that the value of a unit is increasing or decreasing. If it is decreasing then we have to divide and if it is increasing then we have to multiply.

__Complete step-by-step answer:__It is given in the question that 7 workers can build a trench in 25 hours. First we have to find out how many workers will be required to build the same trench in one hour.

We can solve this problem using a unitary method in which we find the value of a unit and then the value of a required number of units.

Always write the things to be found on the right hand side and things known on the left hand side.

Here we have to find out how many workers we need. So, the unknown is the number of workers. We have to put this on the right hand side.

We know about the hours. So, we will take hours on our left hand side.

In 25 hours 7 workers can build a trench.

If we have to finish the same work in one hour we will need more manpower. Therefore,

In 1 hour $7\times 25$ workers can build the trench.

In 35 hours $\dfrac{7\times 25}{35}$ workers can build the trench.

So, we need $=\dfrac{7\times 25}{35}=5$ workers to build the same trench in 35 hours.

Hence, we need 5 workers to build the same trench in 35 hours.

Note: We generally make mistakes when we calculate the value of a unit. We need to understand by reading the question that the value of a unit is increasing or decreasing. If it is decreasing then we have to divide and if it is increasing then we have to multiply.

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