How do objects become negatively charged using the contact method

If a negative object is used to charge a neutral object, then both objects become charged negatively. In order for the neutral sphere to become negative, it must gain electrons from the negatively charged rod. A metal sphere is electrically neutral. It is touched by a positively charged metal rod.

When a negatively charged object comes near a neutral electroscope, it repels the electrons in the neutral electroscope.  

Explanation:

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.