Question 16507
We are trying to find two answers here.  The cost per refrigerator and the overhead cost of the factory per day.

Let's assume the following variables:
r = cost per refrigerator
f = fixed costs (or overhead) per day for the factory

Monday is 30r + f = 25000
Tuesday is 40r + f = 30000

Lets solve for f using Monday
f= 25000 -30r
Now that we know what f is in terms of r let's put it into Tuesday's equation.

40r + (25000 - 30r) = 30000
40r -30r + 25000 = 30000
10r + 25000 = 30000
10r = 5000
r = 500

Plug r into either equation.

30(500) + f = 25000
15000 + f = 25000
f = 10000

or

40(500) + f = 30000
20000 + f = 30000
f = 10000

The answers are :
f (overhead) = 10,000 per day
r (cost per refrigerator) = 500 per unit