The strength of a cable is proportional to the square of its diameter. If a 2-cm cable will support 800kg, how much will a 3-cm cable support?

Strength [is proportional to] d^2

strength1 / (d1) ^2 = strength2 / (d2) ^2

800kg / (2cm) ^2 = strength2 / (3cm) ^2

strength2 = 800 kg * (3 cm) ^2 / (2 cm) ^2

strength2 = 800 kg * 3^2/2^2

strength2 = 800 kg * 9/4

strength2 = 1800 kg