Question

The curved surface of a cylindrical bulb of height $18cm$ and base radius $7cm$ is to be painted in luminating gold colour such that ${\frac{2}{3}}^{rd}$ of the height of the bulb emits golden light; painting is done at ₹ 5 per sq. cm. The cost of painting the bulb is_____.
$(use\pi =\frac{22}{7})$

• A. ₹ 1650
• B. ₹ 2750
• C. ₹ 2640
• D. ₹ 2800

Answer 1

The correct option is C

₹ 2640

Curved Surface area of the bulb
$=2\pi rh$
$=2\times \frac{22}{7}\times 7\times 18$
$=792c{m}^2$

Area to be painted
$=\frac{2}{3}\times 792$
$=528sq.cm$

Hence, total cost of painting
= Area to be painted × Rate of painting
= 528 × 5
= ₹ 2640

[Alternately, we may first calculate ${\frac{2}{3}}^{rd}$ of the height of the cylindrical bulb and then with this height in consideration we may calculate the CSA to be painted, followed by finding the total cost for painting.]