Answer:
The force the construction crew must apply to the lever is 266.67 N
Explanation:
Mechanical advantage is the measure of the ratio of output force to input force in a system. Hence, mechanical advantage (MA) is given by the formula

Where MA is the mechanical advantage
is the output force
and
is the input force
From the question, the mechanical advantage of the lever is 6
That is, MA = 6
and the lever applies a force of 1600 N to the rock
That is, Output force,
= 1600 N
To determine the force the construction crew must apply to the lever, that is the input force, 
From

We can write that

Then,


Hence, the force the construction crew must apply to the lever is 266.67 N.