Answer:
3 square feet per minute
Step-by-step explanation:
The equation
30x + 30y = 450
can be divided by 30 to
x + y = 15
If we multiply this by 20:
20x + 20y = 300
This can be subtracted from the second equation yielding:
5y = 45 and therefore y = 9
Next we can calculate x:
345 - 25 * 9 = 120 = 20 * x
From which we derive x = 6
Therefore the difference is 9 - 6 = 3