What you have to do here is to find a common denominator.
In this case, both 8 and 16 are multiples of 16.
therefore, you've got to multiply the first equation by 2.
1/8*2= 2/16
You've then got to add these together because you've got the same denominator.
2/16+ 3/16= 5/16 (as you don't add the denominators)
If you've meant to put the answer twice in the question, then you've simply got to double it.
Hope this helps :)