In a math word problem, it is common to use a formula. In this case when the math word problem includes absorbance and wave length, then the formula should be used where the absorbance amount is equal to the amount of the molar absorptivity multiplied by the pathlength and multiplied that by the concentration amount. With that said, the problem gives the amount being absorbed, so the answer on the left of the equal side is given at 0.40. The light path is 2 centimeters and it is reduced to 1 cm.
When plugging in the numbers, the answer will come out to be 0.20 based on having to divide 0.40 by the light path amount of two after moving some of the numbers around to get the variable on one side.