6 1a: Basic mathematical techniques ⏐ Part A Basic mathematics
1.5.2 Example: Decimal places
(a) 49.28723 correct to four decimal places is 49.2872
Discarding a 3 causes nothing to be added to the 2.
(b) 49.28723 correct to three decimal places is 49.287
Discarding a 2 causes nothing to be added to the 7.
(c) 49.28723 correct to two decimal places is 49.29
Discarding the 7 causes 1 to be added to the 8.
(d) 49.28723 correct to one decimal place is 49.3
Discarding the 8 causes 1 to be added to the 2.
1.6 Significant figures
Another method for giving an approximated answer is to round off using significant figures. Significant means
important and the closer a digit is to the beginning of a number, the more significant it is.
For example, if we want to express 95,431 to 3 significant figures, '31' will be discarded, leaving 95,400 (3sf).
Zeros have specific rules. All zeros
between non-zeros are significant. For example, 20,606 has 5 significant
figures. Leading zeros in a decimal are
not significant. For example, 0.025 has 2 significant figures.
Question
Significant figures and decimal places
(a) Round off the number 37,649 to one significant figure
(b) Round off the number 0.073184 to one significant figure
(c) Round off the number 0.0073184 to four decimal places
(d) Work out the answer to 974 × 586 on a calculator and round off the answer to three significant figures
(e) Work out the answer to 23 ÷ 946 on a calculator and round off the answer to three decimal places
Answe
(a) 40,000
(b) 0.07
(c) 0.0073
(d) 974 × 586 = 570,764
= 571,000 (3 sf)
(e) 23 ÷ 946 = 0.02431289641
= 0.024 (3 dp)
It is vitally important that you are able to perform calculations correct to a given number of significant figures or
decimal places as correct rounding is essential in computer based assessments. If you did not get all parts of the
above question on significant figures and decimal places correct, work through Section 1 again and retry the
question. Do not underestimate the importance of understanding significant figures and decimal places.
Assessment
focus point