Estimation – JSS1 Mathematics Lesson Note (First Term)

Estimation may be explained as a rough or sensible guess for a value or calculation. Although the estimated value is not exact, it gives us an idea of what the correct answer should be.

Behavioural objectives

By the end of the lesson, students should be able to:

• explain what estimation means;
• name the common units of length, mass and capacity;
• choose the most appropriate metric unit for measuring a given length;
• round off a number to a given number of significant figures;
• round off a number to a given number of decimal places.

What are the common metric units of measurement?

Units of length

• Kilometres (km)
• Metres (m)
• Centimetres (cm)
• Millimetres (mm)

Units of mass

• Tonne
• Kilogram (kg)
• Gram (g)

Units of capacity

• Litre (l)
• Centilitre (cl)
• Millilitre (ml)

How do you choose the most appropriate unit of length?

It is important to be able to choose the most appropriate metric unit of measurement. To measure distances less than a metre, use smaller units such as millimetres (mm) and centimetres (cm). To measure large distances, use metres (m) and kilometres (km).

State the metric unit of length you would use to measure each of the following:

How do you round off a number?

Look at the digit just after the place you are rounding to.

• If it is 0, 1, 2, 3 or 4, change it to 0 (digits after the decimal point are simply dropped).
• If it is 5, 6, 7, 8 or 9, change it to 1 and add it to the next digit on the left.

Here s.f. means significant figures and d.p. means decimal places. For decimal places, we count after the point.

Example 1

Round off 492.763 to (a) 2 s.f., (b) 3 s.f., (c) 2 d.p., (d) 4 d.p.

Solution

1. 492.763 = 490 (the third figure is 2, so it changes to zero).
2. 492.763 = 493 (the 7 changes to 1 and is added to the 2, which becomes 3).
3. 492.763 = 492.76 (the digit after the second decimal place is 3, so it is dropped).
4. 492.763 = 492.7630 (the number has only 3 d.p., so we add a zero).

Evaluation

1. What is estimation?
2. List the standard units of mass and the standard units of capacity.
3. Which metric unit of length would you use to measure the length of a pencil?
4. Round off 492.763 to 1 d.p.
5. Round off 492.763 to 1 s.f.

Assignment

1. State the metric unit of length you would use to measure (a) the width of your exercise book, (b) the distance from your school to your house.
2. Round off 385.472 to (a) 2 s.f., (b) 3 s.f., (c) 2 d.p.