Step by step
The question
Question: Ana has $20 and spends $35.
What is her balance?
1 Write it as a sum
We start at 20.
Spends means down: take away 35.
20 − 35
Money we have is positive.
Money we owe is negative.
Ana has $20: that is 20
spends 35 → − 35
2 Work it out
Start at 20.
Go down 35.
We pass 0.
Careful!
Ana spends more than Ana has.
The balance goes below zero.
Careful: not $35 − $20 = $15 ✗
We land on −$15.
= −$15
So the answer is −$15: Ana owes $15.
3 Check
Let's check: go back.
−15 plus 35 is 20, where we started.
So the answer is right!
−15 + 35 = 20
Let's look back.
First: write it as a sum.
Then: work it out.
Last: check.
Answer
= −$15