precision $0\mathrm{.}01kg,$ then we drop off $6\mathrm{.}052kg$ of potatoes at your laboratory as measured by a scale with

precision $0\mathrm{.}001kg.$ Then, we go home and add $13\mathrm{.}7kg$ of potatoes as measured by a bathroom scale with

precision $0\mathrm{.}1kg.$ How many kilograms of potatoes do we now have and how many significant figures are

appropriate in the answer? The mass is found by simple addition and subtraction:

$7\mathrm{.}56kg-6\mathrm{.}052kg$

$\frac{+13\mathrm{.}7(kg)}{15\mathrm{.}208(kg)}=15\mathrm{.}2kg.$

Next, we identify the least$-$precise measurement: $13\mathrm{.}7kg.$ This measurement is expressed to the $0\mathrm{.}1$

decimal place, so our final answer must also be expressed to the $0\mathrm{.}1$ decimal place. Thus, the answer is

rounded to the tenths place, giving us $15\mathrm{.}2kg.$

Significant figures in this text

In this text, most numbers are assumed to have three significant figures. Furthermore, consistent numbers of

significant figures are used in all worked examples. An answer given to three digits is based on input good to at

least three digits, for example. If the input has fewer significant figures, the answer will also have fewer

significant figures. Care is also taken that the number of significant figures is reasonable for the situation

posed. In some topics, particularly in optics, more accurate numbers are needed and we use more than three

significant figures. Finally, if a number is exact, such as the two in the formula for the circumference of a circle,

$C=2r,$ it does not affect the number of significant figures in a calculation. Likewise, conversion factors such

as $100c\frac{m}{1}m$ are considered exact and do not affect the number of significant figures in a calculation.