Application of Newton's Laws of Motion-Numerical based on Mass connected with String-A

How to slove the numerical problems based on mass connected to string using Newton's Laws of motion?
Application of Newton’s law (Connected Motion): Horizontal Motion:
Solving Problems in Mechanics: For solving problems related to mechanics where a number of forces are involved, we follow following steps;
a) Draw a diagram showing various parts of system with links and choose any one convenient part of the system.
b) Draw a separate diagram of chosen part showing all forces on the chosen part by the remaining part of the system and also by other agencies. This diagram is called free body diagram (FBD).
c) In FBD, include information about magnitude and direction of forces. Equate the net force acting in a particular direction to its mass times the acceleration produced in that direction.
d) Follow the same procedure for other parts of system. The equation of motion obtained for different parts of the system can be solved to obtain the desired result.
Q1.Three blocks A, B and C are placed on a frictionless surface having mass 3kg,2kg and 1kg respectively and force F =12N is applied horizontally as shown in figure. Calculate ( i) acceleration produced in the system of blocks (ii)forced applied by first block on second block (iii) forced applied by second block on third block (iii)Net force on second block.
Q2.Three blocks A, B and C of masses 4kg, 2kg and 1kg are connected by a massless string as shown in figure and placed on a frictionless table and a force of 14N is applied horizontally. Find acceleration of each block (ii) Tension in each string between the blocks.
Q3.The blocks of masses 2kg, 3kg and 5kg are connected by light, inextensible strings are shown in figure. Calculate tension in the string (a) when the system is held stationary (b) Calculate common acceleration and tension in the string, when it is raised upward by applying a force of 300N.
Q4.Two blocks of masses 2kg and 4kg are connected by two identical springs of spring constant 200N/m as shown in figure. Find the extension in both strings.
Please see the Video for better understanding.