Estimating and using a calculator

Last time

Before we start: last time, in Standard form, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Does that look right?

You type a long calculation and the screen shows four thousand. Is that sensible? One slip with a bracket and the answer is wildly wrong. Today you round numbers, estimate any calculation in seconds, and use your calculator with brackets so the real answer is right.

Decimal places

To round to a number of decimal places, look at the digit just after the last one you keep. If it is five or more, round up. If it is four or less, leave the last digit alone. Six point zero seven four nine to two decimal places: the next digit is a four, so it stays: six point zero seven.

Significant figures

Significant figures count from the first digit that is not zero. Leading zeros do not count. Zero point zero zero three zero four seven to two significant figures. The first is three, the second is zero. The next digit is a four, so we stay. The final zero stays too, because it is significant. For big numbers, fill the gaps with zeros. Twenty-four point oh seven to the nearest ten is twenty.