Puzzle | Find the Jar with Contaminated Pills

You are given five jars of pills, each containing the same number of pills. Every pill normally weighs 10 grams, but one jar contains contaminated pills that weigh 9 grams each. You are allowed to use a digital weighing scale only once.

Check if you were right - full answer with solution below.

Solution:

Since we are allowed to use the weighing scale only once, we need to use a smart strategy rather than weighing each jar separately.

Step 1: Label the jars as Jar 1 to Jar 5.

Step 2: Take a different number of pills from each jar:

• 1 pill from Jar 1
• 2 pills from Jar 2
• 3 pills from Jar 3
• 4 pills from Jar 4
• 5 pills from Jar 5

Step 3: Weigh all the selected pills together in one measurement.

If all pills were normal, the total weight would be 150 grams (15 × 10 g). Since one jar has pills that are 1 gram lighter, the actual weight will be less than 150 grams.

Step 4: Identify the jar

• 149 g → Jar 1
• 148 g → Jar 2
• 147 g → Jar 3
• 146 g → Jar 4
• 145 g → Jar 5

Observation: The difference between 150 g and the measured weight directly tells the jar number containing contaminated pills.