An vector of   int named  a that contains exactly five elements has already been declared and initialized. in addition, an   int variable   j has also been declared and initialized to a value somewhere between 0 and 3. 
write a single statement that assigns a new value to the element of the vector indexed by   j . this new value should be equal to twice the value stored in the next element of the vector (i. e. the element after the element indexed by   j ).  do not modify any other elements of the vector!  

The statement for the question is following:-

a[j]=a[j+1]*2;

Explanation:

We have to modify the element at the index j.Since the value of j is between 0 to 3 and the vector contains exactly 5 elements.So j will not exceed the size of vector a.

Now what we have to assign the value at index j is the value double at index next to j that is j+1.So the statement will be

a[j]=a[j+1]*2;

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explanation: