If there are 400 n and there is a mass of 100 kg, what is acceleration?

Several information's that are required to get to the answer of the question is already given in the question. It is important to write them down first and then start solving the problem.
Force = 400 newton
Mass = 100 kg
We already know that
Force = Mass * Acceleration
Then
Acceleration = Force/Mass
                     = 400/100
                     = 4 m / S^2
The acceleration of the object is 4 m/s$2. I hope the procedure is clear for your understanding. In future you can use this method for solving such questions.

Here, we need to set up the equation for acceleration.

400 N/ 100 kg = a

f/m = a

Now, we know that Newtons = kg*m/s^2

400/100 = 4

4 m/s^2 is the acceleration. 

The charge of an ion with 55 protons and 52 electrons is +3. the reason is because protons are positive, and there are three more than the amount of negative electrons.