Question { Microsoft, 15426 }

1) 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 .

Answer

Answer is 1.

We will use a weigh measurement machine.
Initially, it will show weight @ zero. Now start putting coin one-by-one and measure the weight every-time a coin is placed. Where the difference in unit weight is minimum, then that coin will be the lightest.
Eg. when coin 1 is placed- total weight = 30 gms
when coin 2 is placed- total weight = 40 gms (cumulatively)
when coin 3 is placed- total weight = 45 gms (cumulatively)
So the lightest coin is 3rd coin.