Answer: 30 gallons per minute.
Explanation:
In the tank we can see that at the time
t = 0 mins, there are 0 gallons of water in the tank.
t = 1 min, 30 gallons of water in the tank.
t = 2 min, 60 gallons of water in the tank.
t = 3 min, 90 gallons of water in the tank.
The rate of change (or slope) can be obtained as:
s = (y2 - y1)/(x2 - x1)
where y2 = y(x2)
so we can find the slope with two of the previous points:
s = (60gal - 30gal)/(2min - 1min) = 30gal/min
So the rate of cange in the gallons per minute is 30 gallons per minute.