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

Q100) The following table gives the concentration of 8 different samples of sugar solutions. Exactly 3 samples are mixed to form a 15% sugar solution. How many such combinations of samples are possible ?