Digital Sum Method in Simplification: SSC & Bank Shortcut

Last updated: June 27, 2026. For coaching and doubts, contactShortcut Maths, Jadavpur, Kolkata 700032. Call+91-9804490328.

What is the Digital Sum Method?

Digital sum (also called digital root or Navasesh) means you keep adding the digits of a number until only one digit remains.

Example: Digital sum of 4,739 = 4 + 7 + 3 + 9 = 23 → 2 + 3 = 5. Shortcut: Ignore any 9s or combinations that add up to 9. So 4 + 7 + 3 + 9 → drop 9; 4 + 7 = 11 → 1+1 = 2; 2 + 3 = 5.

The 4 Rules You Must Memorise

Traditional Method vs Digital Sum Method

Both match → answer is likely correct.

DS(892) = 8 + 9 + 2 = 19 → 1 (or skip 9: 8 + 2 = 10 → 1). DS(47) = 4 + 7 = 11 → 2. DS(product) = DS(1 × 2) = 2.

DS(5,739) = 5 + 7 + 3 + 9 = 24 → 6. DS(2,814) = 2 + 8 + 1 + 4 = 15 → 6. DS(difference) = DS(6 − 6) = 0 or 9. Actual answer: 2,925 → DS = 2+9+2+5 = 18 → 9. ✓

Exceptions: When Digital Sum Can Mislead You

Same digital sum in multiple options: If every option ends with DS 5, you cannot pick one.
Digit transposition errors: 45 and 54 both have DS 9, so the method won't catch swapped digits.
Extra or missing zeroes: 120 and 102 have the same DS but are different numbers.
When exact value is asked: Digital sum only verifies; it rarely gives the exact answer in algebra-heavy simplification.

Watch out for these traps:

            Same digital sum in multiple options: If every option ends with DS 5, you cannot pick one.
            Digit transposition errors: 45 and 54 both have DS 9, so the method won't catch swapped digits.
            Extra or missing zeroes: 120 and 102 have the same DS but are different numbers.
            When exact value is asked: Digital sum only verifies; it rarely gives the exact answer in algebra-heavy simplification.

Applying Digital Sum in Real Exam Simplification

Question: 28% of 450 + 45% of 280 = ?

Options: (a) 240 (b) 252 (c) 264 (d) 276

Digital sum of LHS: 28% of 450 = 126 → DS = 9. 45% of 280 = 126 → DS = 9. Sum DS = DS(9 + 9) = DS(18) = 9.

Check options: (a) 240 → 6, (b) 252 → 9, (c) 264 → 3, (d) 276 → 6. Only option (b) matches.

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FAQs

If DS(A) < DS(B), add 9 before subtracting. For example, DS(12 − 25) = DS(3 − 7) = DS(3 + 9 − 7) = DS(5) = 5. Actual −13 has DS 1+3 = 4? Wait: DS(−13) is usually taken as DS(13) = 4. This is why digital sum is best used as a quick cross-check, not the primary solver.

It can eliminate wrong options quickly, but for exact answers you still need core arithmetic. Use it as a verification/filter tool.

Try our top 100 simplification questions for SBI PO & IBPS PO.

Last updated: June 27, 2026. For SSC/Bank coaching in Kolkata, call +91-9804490328.

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