The answer must retain the
same number of digits to the right of the decimal point as were present in the
value with the fewest number of digits to the right of the decimal point.

Question: If you
have 3.18 L of water and you add 0.01315 L more. How much water do you end up
with?

Answer: It is easiest
if you look write out the addition problem by aligning the decimal point.