You have 8 coins. 3 of them weigh x units, 3 y units, 1 a units and 1 b units. They are all mixed and look identical. You have to find the lightest coin in minimum number of weighing (balance). Note that you can either use normal balance or digital balance if you like, but we have to consider all the magnitudes of x,y,a,b relative to each other (i.e, x>y>a>b or x<y>a<b are some of the possibilities).