Ah, yes, the good old Circular Lawn or Swimming Pool Question

You may get a question that not many people, outside of a math test, would worry about. Or think about.

The old circular lawn or swimming pool question.

As the incredibly professional-looking graphic above looks more like a pool than a lawn, let's go with this type of question...

A circular pool has a sunbathing deck in the middle. The distance from the centre to the inner edge of the water is 4m. The distance from the centre to the outer edge is 6m. What area does the water cover?

(Hint: It isn't 2 metres.) 

What is missing?

Imagine the pool deck is a like the centre of a pie that has been eaten.The water is the pie that hasn't been eaten. So, work out the area of the whole pie and subtract the area of the piece that was eaten. What is left is your answer.

The area of a circle is...

pi X radius squared

So...

pi x 6 squared         subtract        pi x 4 squared

62.83 square metres.

Note: You won't get the right answer by working out pi x 2 squared (2 being the difference between 6 and 4.)

Perhaps the easiest way to remember this is  

pi x big radius squared  

subtract 

pi x small radius squared