Two identical metallic spheres a and b each carrying a charge of q c are fixed .they repel each other with a force of 2×10-⁵ n. another identical unchanged sphere c is made to touch a, moved to touch b , and placed halfway between a and b. what is the electric charge and the electric force (in n) on each sphere in terms of q?

It really doesn't matter for any of the questions that you've listed.

The boat's velocity relative to the riverbank can be resolved into two components. the component going north is 2.7 m/s and the component going west is 1.2 m/s.we add these two vectors and then find the magnitude of the resultant to get the speed.the magnitude of the resultant vector from adding can be obtained by the pythagorean theorem.    to two significant figures, the boat's speed relative to the riverbank is 3.0 m/s.