A cylindrical bucket of radius 21 cm is filled with water up to a height of 40 cm. How many litres of water is that? Use π = $\frac{22}{7}$ and 1 L = $1000{\text{cm}}^3$.

A cylinder is a circle stacked vertically. Two measurements: radius $r$ and height $h$.

**Curved surface area (CSA) = 2π$rh$** (imagine 'unrolling' the curved surface — it becomes a rectangle of width 2π$r$ and height h).

**Total surface area (TSA) = 2π$r^2+2$π$rh=2$π$r(r+h)$** (curved surface + two circular ends).

**Volume = π$r^2$$h$** (area of circular base × height).

Use π = 22/7 when $r$ is a multiple of 7; otherwise π ≈ 3.14.