Step by step
The question
Question: A submarine is at −40 m.
It sinks 25 m.
Where is it now?
1 Write it as a sum
We start at −40.
Sinks means down: take away 25.
−40 − 25
Sea level is 0.
Below it is negative.
−40 m: 40 m below sea level
sinks 25 → − 25
2 Work it out
Start at −40.
Go down 25.
Careful!
We start at −40, not at 40.
Keep the minus sign.
Careful: not 40 − 25 = 15 ✗
We land on −65 m.
= −65 m
So the answer is −65 m: 65 meters below sea level.
3 Check
Let's check: go back.
−65 plus 25 is −40, where we started.
So the answer is right!
−65 + 25 = −40
Let's look back.
First: write it as a sum.
Then: work it out.
Last: check.
Answer
= −65 m