Rounding & Estimation (Cambridge (CIE) IGCSE International Maths: Extended): Flashcards

When rounding a number to the nearest 100, the digit in the tens place value column determines how the number rounds.

E.g. When rounding 14, 578 to the nearest 100, the 7 in the tens column tells you to round up to 14, 600

The first significant figure of a number is the first non-zero digit of the number when reading from left to right.

For example, the first significant figure of 0.457 is 4.

False.

The second significant figure of 4.0051 is 0.

The number 0 can be a significant figure as long as there are non-zero values on either side of it.

To round a number to three significant figures:

1. Find the third significant figure

2. Check the digit to its right (the fourth significant figure)

   1. If this is 0, 1, 2, 3 or 4 then keep the third significant figure the same and fill the remaining place values with 0s

   2. If it is 5, 6, 7, 8 or 9 then round the third significant figure up to the next value and fill the remaining place values with 0s

E.g. The fourth significant figure of 40 5329 is 3, this means the third significant figure (5) stays the same and it rounds down to 40 500.

When estimating a calculation, the general rule is to round each number to 1 significant figure first.

E.g. For the calculation , round to

If you round both numbers up in an addition, the answer will be an overestimate.

E.g. Estimate the calculation 288 + 962.
Each value rounds up to 1 significant figure, 300 + 1000 = 1300.
(This gives an overestimate 288 + 962 = 1250).

If you round both numbers down in a multiplication, the answer will be an underestimate.

E.g. Estimate the calculation 621 × 438.
Each value rounds up to 1 significant figure, 600 × 400 = 240 000.
(This gives an underestimate 621 × 438 = 271 998).

False.

If you round both numbers up when estimating a division, it is not easy to tell as to whether the the answer be an overestimate or an underestimate.

False.

When estimating a subtraction, a - b, if a is rounded up and b is rounded down, the result will be an overestimation.

E.g. Estimate the calculation 487 - 317.
Rounding each value to 1 significant figure, 500 - 300 = 200.
(This gives an overestimate 487 -317 = 170).