In a proportion, the 1st , 2nd and 4th terms are 51 , 68 and 108, res…

In the given proportion, 

a : b :: c : d

   d = \(\frac{b\; \times\; c}{a}\)

108 = \(\frac{68\; \times\; c}{51}\) 

  c   = \(\frac{108\; \times\; 51}{68}\)

  c   = 81