Well, to solve this problem, we must convert the number and / or the units in a more imaginable one.
We are given two choices:
a. 7.4 x 10^-4 mm
b. 7.4 x 10^4 mm
Converting the units and numbers into meters and general form, respectively. (What is meant by general form is not to write it in scientific notation).
a. 7.4 x 10^-4 mm (1 m / 1000 mm) = 0.00000074 m
b. 7.4 x 10^4 mm (1 m / 1000 mm) = 74 m
Now comparing the two values above, what do you think is the distance between the tracks on a DVD? Definitely NOT 74 m or else the DVD is extremely big. Therefore the correct is the smaller one.
Answer:
7.4 ⋅ 10−4 mm