Define direct, inverse and joint variation.
Direct Variation: where one variable is a constant multiple of another

Inverse Variation: when one of the variables increases, the other one decreases

Joint Variation: where are more than two variables are related directly define direct, inverse and joint variation

Net force due to all on the refrigerator is 400 N

Explanation:

As we know that two people are pushing the refrigerator while other two people are lifting the refrigerator

So net push force on the refrigerator is given as

Similarly net lift force on the refrigerator is given as

Now net force on the refrigerator is given as