A farmer has 9 eggs and thinks one is ligher than the other 8 if he used a balance scale to weigh the eggs only 2 different times how could he find the one egg that weighs less?

This is easy... STEP 1. Put 3 eggs on one side of the scale and put 3 eggs on the other side of the scale. If the scale balances - you know the light egg is not one of the eggs currently on the scale. If the scale tips - you know the light egg is on the "lighter" side. STEP 2. Since Step 1 will tell you which group of 3 the light egg is in... use the scale again... put 1 egg on each side. If the scale balances - the lightest egg is in your hand. If the scale tips - you know the light egg is on the "light" side.