Jar Puzzle

There are multiple methods that can be used to solve this.

Method 1: Algebra

Let x = the weight of an empty jar
Let y = the weight of the contents of a full jar

We know that x + y = 120
Therefore, y = 120 – x

We also know that 2x + 0.5y = 150

Now, let’s plug one equation into the other and solve:

2x + 0.5(120 – x) = 150
2x + 60 – 0.5x = 150
1.5x = 90
x = 60

An empty jar weighs 60 grams.

Method 2: Doubling

Double the right side.
Now you know that 2 empty jars and 2 half-filled jars weigh 300 g in total.
This means that 3 empty jars and 1 full jar weigh 300 g.
From the left scale, you know that 1 full jar weighs 120 g.
You now know that 3 empty jars weigh 300 g – 120 g = 180 g.
Divide this by 3 to determine the weight of 1 empty jar: 60 g.

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