This lesson teaches the mental math habits and shortcuts that improve speed and confidence in case interviews.
You do not need to be a human calculator to do well in cases.
But you do need to:
Mental math becomes much easier when you use structured shortcuts instead of brute force.
Good mental math is mostly about decomposition and approximation.
Strong candidates rarely multiply awkward numbers in one jump. They simplify first.
Examples:
The goal is not to show off. It is to keep the calculation fast and reliable.
Use mental math techniques when:
State your rounding when needed so the interviewer can follow the logic.
Look for clean nearby numbers.
For example:
24 x 18 = 24 x (20 - 2) = 480 - 48 = 432
Examples:
Build from them rather than recalculating from scratch.
Rounding is fine if you are explicit and the decision does not depend on precision.
Sometimes an approximate answer is enough. Sometimes the exact number matters more.
Suppose the interviewer asks:
“What is 18% of 250 million?”
A fast clean method:
That is much better than trying to multiply 250 by 0.18 in one mental step without structure.
A strong candidate:
A weaker candidate often:
Breaking numbers apart is not weakness. It is good process.
If you round, you still need to understand how the adjustment affects the answer.
Be clear, but do not turn every easy step into a lecture.
Small math mistakes happen. What matters is whether you recover cleanly.
Solve these mentally:
Then write the decomposition method you used for each.
A strong way to talk through mental math is:
I will simplify this first. Instead of multiplying directly, I will treat 18 as 20 minus 2, which makes the arithmetic faster and easier to check.
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