A uniform metre ruler is pivoted at its centre. A 3.0 N weight hangs at the 10 cm mark. Where should a 6.0 N weight hang to balance the ruler?
At the 20 cm20\ \mathrm{cm}20 cm mark.
At the 90 cm90\ \mathrm{cm}90 cm mark.
At the 70 cm70\ \mathrm{cm}70 cm mark.
At the 60 cm60\ \mathrm{cm}60 cm mark.