The number that goes into the box marked A is 8
From the input and output values given ;
1 = 2
2 = 4
5 = 10
A = 16
We can see that each input is multiplied by 2 to produce the corresponding output value.
This means that for an output value of 16 .
Input, A = 16/2
A = 8
Therefore, A = 8