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Failed in Stage 1 = 15times |eq 1
Failed in Stage 2 = 12times |eq 2
Failed in Stage 3 = 08times |eq 3
Failed in S1 & S2 = 06times |eq 4
Failed in S2 & S3 = 07times |eq 5
Failed in S1 & S3 = 04times |eq 6
S1,S2 and S3 = 04times | eq 7

We need to find failure in single stages only.

Subtract eq 7 from eq 1 to eq 6
New values:
Failed in Stage 1 = 11times |eq 8
Failed in Stage 2 = 08times |eq 9
Failed in Stage 3 = 04times |eq 10
Failed in S1 & S2 = 02times |eq 11
Failed in S2 & S3 = 03times |eq 12
Failed in S1 & S3 = 00times |eq 13

Subtract eq 13 from 8 and 10
Subtract eq 12 from 9 and 10
Subtract eq 11 from 8 and 9

New values:
Failed in Stage 1 = 09times |eq 8
Failed in Stage 2 = 03times |eq 9
Failed in Stage 3 = 01times |eq 10

This gives us individual values
Adding them 9+3+1 =13 answer. C

Feel free to ask the justification of any step if required.
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