To be able to determine the number of M&Ms that are needed in order to measure the length of Mississippi River, we convert the units to a common one. For this purpose, we convert kilometers to centimeter such that,
Length of Mississippi River = (4000 km)(1000 m/1 km)(100 cm/1 m)
= 400,000,000 cm
Since there are 400,000,000 cm in the entire length of the river, the answer would also be 400,000,000.
