Step 1: Define variables

Let x be the number of fish in one aquarium.
Then 2x is the number of fish in the second aquarium (since there are 2 times more).

Step 2: Formulate the equation

The total number of fish in two aquariums is 12:
x + 2x = 12

Step 3: Solve the equation

Add x and 2x:
3x = 12

Divide both sides by 3:
x = 12 ÷ 3 = 4

Step 4: Find the number of fish in each aquarium

In the first aquarium: x = 4 fish
In the second aquarium: 2x = 2 × 4 = 8 fish

Step 5: Check the correctness of the answer

• Total number of fish: 4 + 8 = 12 ✓
• One aquarium has 2 times more: 8 ÷ 4 = 2 ✓

Answer checked and correct!

Why are other options incorrect?

• 12 and 6: 12 + 6 = 18 ≠ 12
• 3 and 6: 3 + 6 = 9 ≠ 12, and 6 ÷ 3 = 2, but the sum is not 12
• 10 and 5: 10 + 5 = 15 ≠ 12