In a proportion, the 1st , 2nd and 4th terms are 51 , 68 and 108, respectively. What is the 3rd term ?

84 83 82 81

81

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