Maybe because you're finding the "difference" between the two numbers. We could also question why the answer to an addition problem is called sum.
Well, 'sum'body said that's what they should be called a long time ago, that's why we're using that term to call them now.
If the number of days is d and he writes 3 pages for every d, then we can say
3d = 81
Answer:
x = 5
Step-by-step explanation:
using the sine or cosine ratio in the right triangle and the exact value
sin30° = 
, then
sin30° = 
= 
= ( cross- multiply )
2x = 10 ( divide both sides by 2 )
x = 5
