In a fuel economy study, each of 3 race cars is tested using 5 different brands of gasoline at 7 test sites located in different regions of the country. If 2 drivers are used in the study, and test runs are made once under each distinct set of conditions, how many test runs are needed?

210

Explanation:

The number of race cars = N₁ = 3 The number of gasoline brands = N₂ = 5 The number of test sites = N₃ = 7 The number of drivers that participated = N₄ = 2

This study must include each N₁, and it has to be done at each N₃, using each N₂, while carried out by each N₄ ⇒ that means that the total number of possibilities:

N₁ x N₃ x N₂ x N₄ = 3 x 7 x 5 x 2 = 210

The total number of test runs is 210.