Three people are pushing a 500 kg of box in the same direction. applied forces are 30 n, 20 n, and 10 n respectively. if the acceleration of the box is 0.02 m/s2, what is the magnitude of a force created by friction?

The total force applied by the three people is:

This force is pushing toward the direction of the motion, while the  frictional force  points in the opposite direction.
We can write Newton's second law applied to the block: the resultant of the two forces must be equal to the product between the block's mass and its acceleration

We know the mass of the block, m=500 kg, and the acceleration, , so we can find the friction: