Question 1197650
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        As you started your solution, it was absolutely right way. 
        I only do not understand why you stopped there, where you stopped and did not go to the end.



<pre>
The reasoning is very simple and intuitively is obvious.


We should place two terms "m" and "n" between given numbers "a" and "b" in a way that
four terms a, m, n, b form an AP.


So, between the extreme terms "a" and "b", there are three gaps of equal length.

Hence, each gap is {{{(b-a)/3}}}, and therefore


    m = {{{a}}} + {{{(b-a)/3}}} = {{{(3a + b - a)/3}}} = {{{(2a + b)/3}}},

    n = {{{b}}} - {{{(b-a)/3}}} = {{{(3b - b + a)/3}}} = {{{(2b + a)/3}}}.


That's all.  At this point, the solution is complete.
</pre>

Solved.