Puzzle 2

The radius of a circle is 18cm. What is the diameter of this circle?

• 36cm

• 42cm

• 30cm

• 38cm

Answer explanation

To find the diameter of the circle you use the formula 2 x r.

Here the radius is 18cm, so the diameter will be 2 x 18 = 36cm.

Find the radius of the circle with diameter 50in.

• 15in

• 100in

• 25in

• 75in

Answer explanation

To find the radius of a circle whose diameter is known, you will use the formula diameter ÷ 2.

Here the diameter is 50in and so,

50 ÷ 2 = 25

Find the diameter of this circle

• 2.5cm

• 25cm

• 10cm

• 75cm

Answer explanation

Diameter = 2 x radius

Here radius = 5cm

Diameter = 2 x 5 = 10cm.

A cake has a diameter of 18cm. What is the radius?

• 9cm

• 27cm

• 3cm

• 36cm

Answer explanation

Radius = Diameter ÷ 2

Here Diameter = 18cm

Radius = 18cm ÷ 2 = 9cm

The diameter of a circular table is 30m. What is its circumference?

• 94cm

• 94.2cm

• 93.5cm

• 90cm

Answer explanation

To find the circumference of a circle when you have the diameter you use the following formula:

Circumference = πd, where π= 3.14

In this case it will be 3.14 x 30 = 94.2m

Find the circumference of this circle

• 30.5cm

• 30cm

• 31cm

• 31.4cm

Answer explanation

To find the circumference of a circle when you have the radius you use the following formula:

circumference = 2πr, where π = 3.14

Here radius is 5cm and so circumference will be

2 x 3.14 x 5 = 31.4cm

The circumference of a circle is 25.12cm. Find its radius

• 4cm

• 2cm

• 8cm

• 10cm

Answer explanation

When you have the circumference you can find the radius. Circumference = 2πr

Here circumference = 25.12cm

25.12cm = 2 x 3.14 x r

Solve 2 x 3.14 = 6.28.

25.12cm = 6.28 x r

So to find the radius = 25.12 ÷ 6.28 = 4cm.

The circumference of a circle is 37.68m. Find its diameter.

• 10m

• 12m

• 14m

• 21m

Answer explanation

When you have the circumference you can find the diameter. Circumference = πd

Here circumference = 37.68m

37.68m = 3.14 x d

So to find the diameter = 37.68 ÷ 3.14 = 12m.