Step by step
The question
Question: with the scale 1 cm : 10 km, how long is 4 cm on the map for real?
map → real
The scale means: 1 cm on the map is 10 km for real.
1 cm → 10 km
1 Multiply both sides
On the map we have 4 cm.
That is 4 times 1 cm.
So the real length is 4 times 10 km too.
4 cm → 10 × 4 km
We multiply both sides by the same number, 4.
× 4 on both sides
10 times 4 is 40.
So the real length is 40 km.
4 cm → 40 km
2 Check
Let's check: go back.
40 divided by 10 should give the map length, 4.
40 ÷ 10 = 4
We get it again.
So 40 km is right!
Let's look back.
First: multiply both sides.
Last: check.
Answer
4 cm → 40 km