Logical Problems

(31) If you have a 3-gallon jug and a 5-gallon jug, how can you measure exactly 4 gallons of water?

• (a) Fill the 3-gallon jug and pour it into the 5-gallon jug. Fill the 3-gallon jug again and carefully pour it into the 5-gallon jug until the 5-gallon jug is full. You are left with 1 gallon in the 3-gallon jug. Empty the 5-gallon jug and pour the 1 gallon from the 3-gallon jug into it. Now, fill the 3-gallon jug one more time and pour it into the 5-gallon jug. You now have exactly 4 gallons in the 5-gallon jug.
• (b) Fill the 5-gallon jug to the top and pour it into the 3-gallon jug. This leaves you with exactly 2 gallons in the 5-gallon jug. Empty the 3-gallon jug, and pour the 2 gallons from the 5-gallon jug into it. Now, fill the 5-gallon jug to the top again and carefully pour it into the 3-gallon jug until it’s full. You now have exactly 4 gallons in the 5-gallon jug.
• (c) Fill the 3-gallon jug to the top and pour it into the 5-gallon jug. Fill the 3-gallon jug again and pour it into the 5-gallon jug until the 5-gallon jug is full. You are left with 1 gallon in the 3-gallon jug. Empty the 5-gallon jug and pour the 1 gallon from the 3-gallon jug into it. Now, fill the 3-gallon jug one more time and pour it into the 5-gallon jug. You now have exactly 3 gallons in the 5-gallon jug.
• (d) It’s impossible to measure exactly 4 gallons with a 3-gallon jug and a 5-gallon jug.

Answer

Correct Answer: (b) Fill the 5-gallon jug to the top and pour it into the 3-gallon jug. This leaves you with exactly 2 gallons in the 5-gallon jug. Empty the 3-gallon jug, and pour the 2 gallons from the 5-gallon jug into it. Now, fill the 5-gallon jug to the top again and carefully pour it into the 3-gallon jug until it’s full. You now have exactly 4 gallons in the 5-gallon jug.
This classic puzzle can be solved using the method described in option (b).

(32) If a doctor gives you 5 pills and tells you to take one pill every half-hour, how long would it take before you’ve taken all the pills?

• (a) 1 hour
• (b) 2 hours
• (c) 2.5 hours
• (d) 2 hours and 30 minutes

Answer

Correct Answer: (d) 2 hours and 30 minutes
If you take one pill every half-hour, and you have 5 pills, it would take you 2 hours and 30 minutes to take all the pills.

(33) If you have a square room and there are four corners in it, and in each corner, there is a cat, how many cats are there in total?

• (a) 1 cat
• (b) 2 cats
• (c) 3 cats
• (d) 4 cats

Answer

Correct Answer: (d) 4 cats
There are four corners in the square room, and in each corner, there is a cat. Therefore, there are 4 cats in total.

(34) What has keys but can’t open locks and can sing but can’t talk?

• (a) A piano
• (b) A computer
• (c) A typewriter
• (d) A telephone

Answer

Correct Answer: (c) A typewriter
A typewriter has keys for typing but can’t open locks, and it can make sounds (sing) as you type but can’t talk like a human.

(35) If you have three apples and you take away two, how many apples do you have?

• (a) 1 apple
• (b) 2 apples
• (c) 3 apples
• (d) 0 apples

Answer

Correct Answer: (b) 2 apples
If you have three apples and you take away two, you still have 2 apples left.