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Estimation & Sanity Checks

Always verify your answer is in the right ballpark.

When to use

After every calculation — build the habit of checking.

Steps

  1. 1.Round both numbers to simple values
  2. 2.Quick-calculate the estimate
  3. 3.Compare: is your answer within 10-20% of the estimate?

Example: Verify: 35% of 7,190 = 2,516.5

Estimate: 35% of 7,000 = 2,450

Our answer 2,516.5 is close to 2,450 ✓

Difference is about 2.7% — within bounds

Mental approach

After calculating 8,430 x 4 = 33,720, check: 8,000 x 4 = 32,000. Close? Yes ✓

Pen & paper approach

Write a quick order-of-magnitude check in the margin.

Tips

  • •Always do this in case interviews — interviewers notice
  • •Round to 1 significant figure for the quickest check
  • •If your answer is off by 10x, you dropped a zero somewhere

Try it yourself

Is 6,400 x 7 = 44,800 reasonable?

Is 80% of 88,400 = 70,720 reasonable?

Is 3,840,000 / 4 = 96,000 reasonable?