The big ideaA rate says how much of one money buys another, and it can change.
How it works
Ines lives in Lumo, where money is a gold star coin. The kite she wants costs six tiles in Pell, so she needs tiles.
The booth's sign says one tile costs three star coins. That is an exchange rate: how much of one money buys one of another.
A rate works both ways. To turn tiles into stars, multiply by three. To turn stars into tiles, divide by three.
Rates change. A week later one tile cost four stars, so the same kite cost six more stars. Real booths keep a little for their work, and real rates change every day.
Which kite is cheaper depends on the rate.Week one and week two.
Example 1: Stars to tiles
A tile costs 3 stars. How many tiles do 15 stars buy?
15 ÷ 3 = 5
15 stars buy 5 tiles.
Example 2: Tiles to stars
Six tiles at 3 stars each.
6 × 3 = 18
Six tiles cost 18 stars, so Ines has 24 − 18 = 6 stars left.
Example 3: A new rate
A week later one tile costs 4 stars.
6 × 4 = 24
Six tiles now cost 24 stars. Only the rate changed.
Example 4: Which place is cheaper?
A kite in Lumo costs 21 stars.
In week one the Pell kite costs 18 stars, so Pell is cheaper.
In week two it costs 24 stars, so Lumo is cheaper.
24 − 21 = 3
Pell is 3 stars dearer in week two.
Words to know
exchange rate
How much of one money buys one of another.
tile
The money used in Pell.
star coin
The money used in Lumo.
booth
Where one money is swapped for another.
Watch out
Check which way you are going: multiply for tiles into stars, divide for stars into tiles.
The kite itself did not change. Only the rate did.
Having more coins does not settle it. A rate decides what a money buys.
A real booth keeps a little for its work, so swapping back gets slightly less.
Try it
At 3 stars a tile, how many tiles do 24 stars buy?
At 4 stars a tile, how many stars do 3 tiles cost?
The sign changes from 3 stars to 4 stars a tile. Did the kite change?
Going from tiles into stars, do you multiply or divide?
Answers: 1. 8 · 2. 12 · 3. No, only the rate did · 4. Multiply
Try at homeAsk a grown-up if they have ever seen an exchange rate on a sign, or swapped money for a trip. What did it say?
For grown-upsIf you have seen an exchange rate on a sign, a screen or a trip, look at it together and try it both ways: multiply to go one way, divide to go back. Real rates change every day.
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