The correct answer is:
$30.
Explanation:
Writing the transaction amounts as signed integers, we have:
+30, -20, -30, +40, -50
We want to add these together. Addition is commutative, which means we can add the positive integers before the negative:
30+40+(-20)+(-30)+(-50)
70+(-20)+(-30)+(-50)
We can now add the negative integers together:
70+(-50)+(-50)
70+(-100)
Adding these together, we have
-30
This means we would need at least $30 to avoid overdrafting the account.
