5 11 6 9
13 18 11 20
21 25 16 ?
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Answer is 31.
How?
Look at columns, difference between second column number to first column number is same as difference between third column number and second column number, i.e.
How?
Look at columns, difference between second column number to first column number is same as difference between third column number and second column number, i.e.
c[i+1] – c[i] = c[i+2] – c[i]
lets assume missing number is X, thus
20 – 9 = X – 20
X = 40 -9 = 31
