Question

Circular discs of $5mm$ thickness, are placed one above the other to form a cylinder of curved surface area $462c{m}^2$. Find the number of discs, if the radius is $3.5cm$.

Answer 1

Thickness of each disc $=5mm=\frac{5}{10}cm=0.5cm$
Radius of disc $=3.5cm$.
Curved surface area of cylinder $=462c{m}^2$
$\therefore 2\pi rh=462$ ....... (i)
Let the no of discs be $x$
$\therefore$ Height of cylinder,$h=$ Thickness of disc $\times$ no of discs$=0.5x$.......(ii)
From (i) and (ii) we get
$2\times \frac{22}{7}\times 3.5\times 0.5x=462$
$\therefore x=\frac{462\times 7}{2\times 22\times 3.5\times 0.5}=42discs$