If he babysits for 2 hours at $7 per hour, he will earn 2 * 7 = $14. This week he will earn $5 allowance, $10 mowing the lawn and $14 from babysitting for a total of 5 + 10 + 14 = $29. If we add the $29 he will earn to his $65 savings, he will have a total of 29 + 65 = $94. The bike costs $100 and he will have $94 leaving $100 - $94 = $6 more that he will need to earn.
The answer: 6
Reverse engineering the question: Zach is saving his money to buy a brand new bike that costs $100. His weekly allowance is $5. His parent will pay him an extra $10 to mow the lawn. His neighbor will pay him $7 per hour to babysit their son. He has already saved up $65. He'll receive his allowance on Friday and he's planning on babysitting for 2 hours this Saturday after he mows the lawn. How much more money does Zach need to earn before he can buy the bike?

The birds eat this many tomatoes: 21 / 3 = 7 tomatoes. There are this many tomatoes left on the plant: 21 - 7 = 14 tomatoes.
The answer: 14
Reverse engineering the question: There are 21 cherry tomatoes on the tomato plant. 2 birds eat one-third of the tomatoes. How many are still left on the tomato plant?

Rob hunts 1 / 2 * 6 animals / day = 3 animals / day. Rob and Sam hunt 3 animals / day + 6 animals / day = 9 animals / day. Mark hunts 1 / 3 * 9 animals / day = 3 animals in a day. Peter hunts 3 * 3 animals / day = 9 animals in a day. The total they all hunt in a day is 6 animals / day + 3 animals / day + 3 animals / day + 9 animals / day = 21 animals.
The answer: 21
Reverse engineering the question: In a day, Sam hunts 6 animals. Rob hunts half as many animals as Sam. Mark hunts 1/3 of the total of what Rob and Sam hunt. If Peter hunts 3 times as many animals as Mark does, how many animals do they all hunt in a day?

If 10 hens lay 80 eggs, then 1 hen lays 80 / 10 = 8 eggs in 10 days. That means one hen is laying 8 / 2 = 4 eggs in 5 days. So in 15 days, 1 hen will lay 3 times more eggs, so 4 * 3 = 12 eggs. All hens in total will then lay 25 * 12 = 300 eggs in 15 days.
The answer: 300
Reverse engineering the question:
Martin owns a farm with hens. 10 hens do lay 80 eggs in 10 days. Martin decided to buy 15 more hens. How many eggs will all hens lay in 15 days?