His new computer uses 800 * .5 = 400 watts more than his old computer. So it users 800 + 400 = 1200 watts. That is 1200 / 1000 = 1.2 kw. Electricity cost 12 * .25 = 3 cents more than it used to. So it now cost 12 + 3 = 15 cents per kwh. He used 1.2 * 50 = 60 kwh. So he paid 15 * 60 = 900 cents per kwh. So he paid 900 / 100 = $9.
The answer: 9
Reverse engineering the question: The price of electricity went up by 25%. John's old computer used 800 watts and his new computer uses 50% more. If the old price of electricity was 12 cents per kilowatt-hour, how much does his computer cost, in dollars, to run for 50 hours?

In 3 years, the sum of their present ages will be added by 3 (from Allen) + 3 (from her mother) = 6. So, the sum of their present ages is 41 - 6 = 35. Let x be the present age of Allen's mother and x - 25 be the present age of Allen. So the equation using the sum of their present ages is x + x - 25 = 35. By combining like terms, the equation becomes 2x = 60. Hence, Allen's mother is x = 60 / 2 = 30 years old now.
The answer: 30
Reverse engineering the question: Allen is 25 years younger than his mother. In 3 years, the sum of their ages will be 41. What is the present age of Allen's mother?

Sydney is 20 * 3 = 60 years old. Mason's father is 60 + 6 = 66 years old.
The answer: 66
Reverse engineering the question: Mason is three times younger than Sydney and Sydney is six years younger than Mason's father. If Mason is 20 years old, how old is his father?

Maria has $120 and received $250 from her mother. So she has 120 + 250 = $370. As the price of the bike stands at $600, Maria needs 600 370 = $230 more to buy the bike.
The answer: 230
Reverse engineering the question:
Maria wants to buy a brand new bike. The retail price at the bike shop stands at $600. She saved $120 toward the purchase. As this was not enough, she asked her mother to give her the remaining amount. Her mother offered her $250 and told her that she needs to earn the rest working during the holidays. How much money must Maria earn to be able to buy the bike she wants?