Physics quiz question: addition/subtraction rule

Physics 5A Quiz 1, spring semester 2008
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 1/e, Problem 1.16

Evaluate the following calculation, using an appropriate number of significant figures and/or decimal places.

66.0 kg + 12.74 g = ?

(A) 66.0 kg.
(B) 66.01 kg.
(C) 66.01274 kg.
(D) 78.7 kg.

Correct answer: (A)

12.74 g is 0.01274 kg, with five decimal places. However, when added to 66.0 kg, which has only one decimal place, the result is 66.01274 kg (response (C)), which must be rounded to one decimal place (using the addition/subtraction rule, that restricts decimal places to the value with the least amount) to become 66.0 kg! Response (B) is rounded to two decimal places, while response (D) is the result of not converting 12.74 g to kg before adding.

Student responses
Sections 4987, 4988
(A) : 24 students
(B) : 5 students
(C) : 3 students
(D) : 18 students