Find out odd one bag with single measurement?

Question

We have 4 bags and each bag balls of 100 gm except one bag which has all 90gm balls.
Now we need to find out the bag which has all 90gm balls with single measurement?

Answer 1

Take out different no of balls from each bag and find the sum for eg: 1 ball from first bag, 3 ball from second bag, 5 balls from third bag, 4 balls from fourth bag.
Now in ideal case if all balls are of same size their weight should be (1+3+5+4) * 100 = 1300 but it is not

If first bag is of 90 gm ball weight will be 1300 - 1*10 = 1290
If second bag is of 90 gm ball weight will be 1300 - 3*10 = 1270
If third bag is of 90 gm ball weight will be 1300 - 5*10 = 1250
If fourth bag is of 90 gm ball weight will be 1300 - 4*10 = 1260

In simple terms substract the actual weight from ideal weight and divide by 10 you will get the no of 90 gm balls & then the corresponding bag.