Question

IB exams

A rope can make 70 rounds of the circumference of a cylinder whose radius of the base is 14cm. how many times can it go round a cylinder having radius 20 cm?

Answer 1

length of rope = 70 × 2πr = 70×2×π×14 = 140π×14

if the cylinder had radius 20, let the number of rounds = y
y × 2πR = 140π × 14

⇒ y = $\frac{140 \pi \times 14}{2 \pi \times 20} =49$

So it can go 49 times round the cylinder.