Answer :

2 / 4 = 1 / 2 ;
2b / 5 = 4b / 10 ;
1 / 2 = 5 / 10 ;
5 / 2 = 25 / 10 ;
4b / 10 + 5 / 10 = 25 / 10 ;
( 4b + 5 ) / 10 = 25 / 10 ;
4b + 5 = 25 ;
4b = 20 ;
b = 20 ÷ 4 ;
b = 5 .