One approach of solving the problem is using summation formula.

1)Calculate the sum of 20 elements by using a for loop from n+10  to n+110 and use the formula of n(n+1)/2 to do it and assign it to variable say s.

2)For finding missing value traverse the array from start to end and update s1 as s1+= a[i]

3)Hence the missing integer will be s-s1 and you can print the value now.