Step by step
The question
Question: change $40 to euros, when $1 = €0.90.
$1 = €0.90. Change $40 to euros.
1 Write the rate
The rate says: 1 dollar is worth 0.90 euros.
$1 = €0.90
From dollars to euros, we multiply by 0.90.
Going back, we divide by 0.90.
dollars → euros: × 0.90
euros → dollars: ÷ 0.90
2 Multiply by the rate
Each dollar is worth €0.90.
40 dollars are worth 40 times as much.
$40 = 40 × €0.90
40 times 0.90 is €36.
= €36
To multiply decimals: first 40 times 90, as whole numbers.
That is 3600.
Then count the decimal places: 2.
40 × 90 = 3600
2 decimal places: 36
Careful!
Check the direction.
Dollars to euros: we multiply by the rate.
Careful: dollars → euros: × the rate
So $40 is €36.
3 Check
Let's check: change the answer back.
€36 divided by 0.90 is $40.
change it back: €36 ÷ 0.90 = 40
That is what we started with.
So the answer is right!
$40 — what we started with
Let's look back.
First: write the rate.
Then: multiply by the rate.
Last: check.
Answer
Careful: dollars → euros: × the rate