We can use the concept of commutative property of addition here. We know that the addition follow the commutative property.
The commutative property of addition is given by
It means, if we change the order, it will not affect the result.
Therefore, using above mentioned property, we can see that
1+7 = 7+1
Hence, if we know the value of 1+7, then we can easily find the value of 7+1 because both are same.