Answer:
There are 887.5 number of excess electrons on the each spheres.
Explanation:
It is given that, the force of repulsion between two spheres is, 
We know that the force of attraction or repulsion between charges is given by :

It is assumed that the separation between the spheres is 20 cm or 0.2 m. So,


Let there are n excess electrons that must be present on each sphere. It can be given by using quantization of charge as :



n = 887.5 electrons
So, there are 887.5 number of excess electrons on the each spheres. Hence, this is the required solution.
A pure substance is only composed of one element for example pure gold which as the only element present Gold (Ag). So the properties of these kinds of substances is also dependent on only one element e.g. ductility, conductance etc.
On the other hand mixture is made of many different pure elements such as Bronze. This gives an object different properties. So mixtures contain many different pure elements
W=F*s
W=9000*0
W=0
W:Work
F:Force
s:Distance
It will be a straight horizontal line on the graph
8 m/s
Explanation:
Using conservation of momentum :-

Where:
m1 = Mass of first vehicle
m2 = Mass of second vehicle
u1 = initial speed of first vehicle
v1 = initial speed of second vehicle
u2 = Final speed of first vehicle
v1 = Final speed of second vehicle
From the received informations:



So

Now divide both sides by m1 :-


Therefore, final answer is 8 m/s