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Answer is 31.

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