Explanation for Question 31 From the Math (Calc) Section on the Official Sat Practice Test 4
So now here for question number 31, 2 we have a partially filled pool containing 600 gallons of water. 3 Right? And so the hose is turned on and water is filling the pool 4 at eight gallons per minute. So we need to know how many gallons of 5 water will it be in the pool after 70 minutes? 6 Right? So we're starting with the time period of 70 minutes, 7 70 minutes have gone by. And we know that every minute for every 8 one minute, we're going to put in eight gallons of, 9 um, of water, right? So really what this is, 10 is a question about unit analysis. 11 We need to take our units of minutes and convert them to gallons by 12 taking a conversion factor right here, 13 putting minutes in our denominator. So the minutes cancel out, 14 leaving us with units in gallons. 15 So if we do that 70 times eight, we get, 16 um, 560 gallons. 17 Um, but that's not just what we have, 18 right? Cause the pool doesn't, um, hold 600 gallons. 19 When it says that the pool contains 600 gallons, 20 it's like the pool is already partially filled and that's at our start, 21 right? So we have the 560 gallons that we added, 22 but we also started with 600 gallons. 23 When we add those together, we get 1,160, 24 which is going to be our answer combining both the starting point and the 25 gallons that we added from the hose.