Chance Is a Fraction

Lesson 1 of 11 · 5 min · Free, no account needed

What you will learn

A probability is the share of the wheel that wins: 2 red slices of 10 is 2/10 = 0.2.
Chances live on a scale from 0 (never) to 1 (always). 0.2 is the same as 20% and 1 in 5.
The number of slices does not matter on its own. The share does: 3 of 15 is also 1 in 5.

A spinner gives a rare skin on red.
How big should red be for a 1 in 5 chance? Drag the edge of the red wedge to your bet.
Paint the wheel so red wins 1 in 5 spins.
The wheel has 10 equal slices. Tap a slice to paint it red. Tap again to clear it.
Every slice is equally likely to stop under the pointer. If 2 of the 10 slices are red, red wins 2 times in 10, which is 2/10 = 1/5 = 0.2.
Spin your wheel a hundred times.
Does red really come up about 1 time in 5?
That share is called a probability.
Probability = winning slices / all slices, when every slice is equally likely. Here 2/10 = 0.2.
It lives on a scale from 0 (never happens) to 1 (always happens).
The same chance has three names: 0.2 = 20% = 1 in 5.
Three red slices. Change the wheel, not the paint.
Turn the dial to cut the wheel into more or fewer slices. Hit each goal.
A chance is a fraction: 3/6 = 1/2, 3/12 = 1/4 and 3/15 = 1/5. Multiplying the top and bottom by the same number keeps the share.
Game: run the spinner shop.
Three orders. Pick a paint, tap slices, Deliver. Use as few taps as you can.
Put every chance on the line.
Drag each card to where its chance sits, from 0 (never) to 1 (always). Two of them are your wheel.
A chance of 0 means never, 1 means always. Red at 0.2 and not red at 0.8 add up to 1, because a spin always lands on one or the other.

Lessons are interactive and need JavaScript. Turn it on to play them.