Question : Let ABCDEF be a prism whose base is a right-angled triangle, where sides adjacent to 90$^{\circ}$ are 9 cm and 12 cm. If the cost of painting the prism is INR 151.20, at the rate of 20 paise per sq cm then the height of the prism is:
Option 1: 17 cm
Option 2: 18 cm
Option 3: 15 cm
Option 4: 16 cm
Correct Answer: 18 cm
Solution :
Total surface area of prism = $\frac{\text{Total cost of painting}}{\text{Cost per sq cm}}=\frac{151.2}{0.2}$ = 756 cm
2
Hypotenuse of the triangular base = $\sqrt{9^2 + 12^2}$ = $\sqrt{81 + 144}$ = $\sqrt{225}$ = 15 cm
Perimeter of base = 9 + 12 + 15 = 36 cm
Let h be the height of the prism.
Total surface area = perimeter of base × height + 2 × area of base
756 = 36 × h + 2 × $\frac{1}{2}$ × 9 × 12
⇒ 756 = 36h + 108
⇒ h = $\frac{756-108}{36}$ = $\frac{648}{36}$ = 18 cm
Hence, the correct answer is 18 cm.
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