The lake is 20 miles across because 2 x 10 = 20. If takes .5 hours to go from bottom to top because 30 / 60 = .5. The lake is five miles from bottom to top because 10 x .5 = 5. The square mileage is 100 because 20 x 5 = 100.
The answer: 100
Reverse engineering the question: A boat is traveling across a square lake at 10 MPH. It takes 2 hours to go across the length of the lake. It then travels the whole width of the lake at the same speed, which takes 30 minutes. How many square miles is the lake?

The total marks Amaya scored more in Music than in Maths is 1 / 10 * 70 = 7 marks. So the total marks she scored in Maths is 70 - 7 = 63 marks. If she scored 20 marks fewer in Maths than in Arts, then he scored 63 + 20 = 83 in Arts. If she scored 10 marks more in Social Studies than in Music, then she scored 70 + 10 = 80 marks in Social Studies. The total number of marks for all the subjects is 70 + 63 + 83 + 80 = 296 marks.
The answer: 296
Reverse engineering the question: Amaya scored 20 marks fewer in Maths than she scored in Arts. She also got 10 marks more in Social Studies than she got in Music. If she scored 70 in Music and scored 1/10 less in Maths, what's the total number of marks she scored in all the subjects?

Jack is able to carry two 5 - gallon buckets of water per trip, or 2 * 5 = 10 gallons of water. Jill can only carry one bucket per trip, or 5 gallons. Jack completes 3 trips to the well and back for every 2 trips Jill completes, so in that time they have both together carried 3 Jack trips * 2 buckets per Jack trip * 5 gallons per bucket + 2 Jill trips * 1 bucket per Jill trip * 5 gallons per bucket = 30 gallons + 10 gallons = 40 gallons of water. If you consider these as a single "group" of trips, it will take 600 gallons / 40 gallons per "group" of trips = 15 groups of trips. As each of these "groups" of trips consists of two Jill trips, Jill will make 15 * 2 = 30 trips to the well and back before the tank is filled.
The answer: 30
Reverse engineering the question: Jack and Jill went up the hill to fetch water to fill a 600 gallon tank. Each bucket is five gallons. Jack is stronger, and is able to carry two buckets of water at a time, but Jill can only manage one. Also, Jack can complete three trips to the well and back in the time it takes Jill to make two trips. How many trips will Jill make before the tank is filled?

The factory creates 300 gummy bears a minute, which means it produces 300 / 50 = 6 packets of gummy bears per minute. It would take 240 / 6 = 40 minutes to manufacture enough gummy bears to fill 240 packets.
The answer: 40
Reverse engineering the question:
The gummy bear factory manufactures 300 gummy bears a minute. Each packet of gummy bears has 50 gummy bears inside. How long would it take for the factory to manufacture enough gummy bears to fill 240 packets, in minutes?