If the fourth term in the expansion of (ax+1/x)n is 20/27 then find the value of n and a . Also find its 4th term from the end

Dear Student,

Please refer the following link for answer to similar query

https://www.meritnation.com/ask-answer/question/if-4th-term-in-the-expansion-of-ax-1-x-n-is-5-2-then-the/binomial-theorem/3335868

once we get the value of a and n we can very easily find the value of 4th term from the end as 
${}^{n}C_{n-3}\times a^3\times x^3\times {\left(\frac{1}{x}\right)}^{n-3}$

Regards,