Math · Stage 1

Reason from the worst case to be sure of a matching pair

In the dark you cannot see colours — so take one more than the number of colours, and a matching pair is guaranteed.

✦ Math📘 Stage 1 · S1⏱ ~9 min🧒 Ages 5–6★ Free to start

I can reason from the worst case: to be sure of a matching pair, take one more than the number of colours.

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Kavi's Sock Drawer in the Dark 🧦

Kavi's socks come in just 2 colours — red and blue — all jumbled in a drawer, and the light is off! "How many socks must I grab to be SURE I have a matching pair?" wonders Kavi. Kavi imagines the unluckiest grab: one red, then one blue — still no pair! But the very next sock must match one of them. So 2 colours + 1 = 3 socks is guaranteed to make a pair.

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What your child will learn

1First, count the coloursKavi's drawer has socks in 2 colours. In the dark you cannot tell which sock is which — you can only know how many colours are possible. Tap each card.

2Imagine the unluckiest grabPretend you are unlucky: the 1st sock is red, the 2nd is blue — still no pair! Sort these three socks into colour bins and watch the 3rd sock: it has to be red OR blue, so it joins a bin and makes a matching pair.

3The rule as a bond: colours + 1Write the rule as a number bond: the number of colours, plus 1 more, equals the socks you need. For 2 colours: 2 + 1 = 3.

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A peek inside

Kavi's drawer has red and blue socks (2 colours). How many socks must Kavi grab in the dark to be SURE of a matching pair?

worst case = one of each colour = 2 socks, still no pair · the next sock must match → 2 + 1

Try this at home: Socks come in 2 colours in a dark drawer. How many must you grab to be sure of a matching pair? Why?

Ready to play and learn?

Start the free interactive lesson, or print the handout for screen-free play.

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Part of Math · Stage 1 · S1 on KAVI Academy — a secular, whole-child learning platform for ages 5–12.