Line OE = radius - depth

Line OA, the tank's radius is 50 cm and line OE is 30 cm.

arc cosine ∠ AOE = OE ÷ OA = 30 ÷ 50 = .6

∠ AOE = 53.13°

∠ AOB = 53.13° * 2 = 106.26°
Sector Area = (106.26 ÷ 360) • 7,853.98 cm² = 2,318.23 cm²

Next we calculate the red triangle area.

Line AE^{2} = radius^{2} -Line OE^{2}

Line AE^{2} = 50^{2} -30^{2}

Line AE^{2} = 2,500 -900

Line AE^{2} = 1,600

Line AE = 40

Area of Red Triangle = Line AE • Line OE

Area of Red Triangle = 40 • 30 = 1,200 cm²

Finally, we get to calculate the segment area.

Segment Area = Sector Area -Red Triangle Area

Segment Area = 2,318.23 cm² - 1,200.00 cm²

Segment Area = 1,118.23 cm²

Volume at 20 cm = Segment Area ÷ "End" Area • Total Volume

Volume at 20 cm = 1,118.23 ÷ 7,853.98 • 1,570,796 cm³ =

Volume at 20 cm = 0.1423779213 • 1,570,796 cm³ =

Volume at 20 cm = 223,647 cm³