The force of friction is 330 N
Explanation:
We can solve this problem by applying Newton's second law of motion, which states that the net force acting on an object is equal to the product between the mass of the object and its acceleration:

where
is the net force
m is the mass
a is the acceleration
For the crate in this problem, we have 3 forces acting on it:
- The force applied by the 1st child,
forward
- The force applied by the 2nd child, 
- The force of friction,
, acting backward
So the net force is

We also know that the crate remains stationary, which means that its acceleration is zero:
a = 0
Therefore, we can rewrite Newton's second law as

From which we find

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