A verandah is held up by identical cylindrical pillars, each 3.5 m tall and 0.4 m in diameter. Only the curved surfaces were painted, and the painted area came to $61.6{\text{m}}^2$ in all. How many pillars are there? Take π = 22/7.

One pillar's radius is 0.2 $m$, so its curved surface = 2πrh = $2\times (22/7)\times 0.2\times 3.5=4.4m^2$. The number of pillars = 61.6/4.4 = 14. (Using the diameter 0.4 $m$ as the radius doubles each pillar to $8.8{\text{m}}^2$ and gives 7; using πrh instead of 2πrh halves it to $2.2{\text{m}}^2$ and gives 28; dividing 61.6 by the bare product $0.4\times 3.5=1.4$ gives 44 and skips π altogether.)