1/2 x 1/3 + 1/3 x 1/4 + 1/4 x 1/5 + 1/5 x 1/6
=.........................................................................................................
=.........................................................................................................
1/2 x 1/3 + 1/3 x 1/4 + 1/4 x 1/5 + 1/5 x 1/6
= 1/3x( 1/2+1/4) + 1/5x(1/4+1/6)
= 1/4 + 1/12
= 1/3