Step 1: Find the 2 base areas and add.
20 x 10 = 200 cm²
30 x 10 = 300 cm²
200 + 300 = 500 cm²

Step 2: Find the volume of water.
20 x 10 x 10 = 2000 cm³

Step 3: Since the volume of water did not change, we divide the volume of water by the sum of base areas to find the height of water in both tanks in the end.
2000 ÷ 500 = 4 cm

Step 4: Find the volume of water in Tank A in the end.
20 x 10 x 4 = 800 cm³

Step 5: Subtract volume of water at first and in the end.
2000 - 800 = 1200 cm³