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Answer:
(a) 10 cm
Step-by-step explanation:
Since C is the midpoint of AB, the triangle shown is 1/4 of the rectangular area shown. When the tank is sitting horizontally on its 100 cm side, the water height will be 1/4 of the 40 cm height of the tank. It will be 10 cm.
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More elaborate explanation
Triangle area = 1/2(AD)(AC) = 1/2(AD)(1/2(AB)) = 1/4(AD)(AB)
Whole tank area = (AD)(AB)
So, the triangle area is 1/4 the whole tank area. When the tank is tipped back to horizontal, the water area will still be 1/4 the whole tank area, so will be ...
water area = (1/4(AD))(AB)
and the height will be 1/4(AD) = 1/4(40 cm) = 10 cm
