Identify which one of these are exact numbers. (Can be more than 1)

a. 2.54 cm (the number of centimeters in exactly 1 in)

b. 56.00 s (a time measured by a stopwatch)

c. 4 choices (the number of choices in this list)

d. 3 lbs (the weight of an object measured on a scale)

Answer: a. 2.54 cm (the number of centimeters in exactly 1 in) and c. 4 choices (the number of choices in this list)

Justification:

1) a. 2.54 cm (the number of centimeters in exactly 1 in)

Although the exact length of one inch has varied in the past, currently the inch is defined as exactly 2.54 cm, so this is an exact number.

b. 56.00 s (a time measured by a stopwatch)

The time measured by a stopwatch is not an exact number. The number is limited by the precision of the instrument (the stopwatch) and it migh have been 56.004 instead of 56.00, and the stopwatch cannot shoe that. So, this is not an exact number.

c. 4 choices (the number of choices in this list)

This is, indeed an exact number. It is 4, not 4.1, nor 3.9, nor any other number.

d. 3 lbs (the weight of an object measured on a scale)

As reasoned with the stopwatch, this measure, 3 lbs, is subject to the precision of the scale. It is not an exact number. It could be other number close to 3: 2.99 for example.