1/x + 1/y = 1/n=> xy = nx + ny=> (x - n)(y - n) = n²
Now, n² must have 5 factors to get 5 ordered pairs=> n must be of form p², where p is prime number
Hence all primes less than 20

@hemant_malhotra said in Question Bank - Algebra - Hemant Malhotra:
Let a < b < c be the three real roots of the equation √2014 x^3 - 4029 x^2 + 2 = 0. Find b(a + c).
https://artofproblemsolving.com/wiki/index.php/2014_AIME_I_Problems/Problem_9

Q91) A four-letter code has to be formed using the alphabets from the set (a, b, c, d) such that the codes formed have odd number of a’s. How many different codes can be formed satisfying the mentioned criteria?