If a changes to b, then doesn't a + d = b? Why a(1 + d) = b? [closed]
Solution 1:
The $d$ in that equation is not the amount of change.
If $a=2$ apples and you buy $3$ more then you do have $5$ and the change is indeed $3$.
The relative change is $5/2 = 2.5$. You have two and a half times as many apples, or a $150\%$ change.
In the original formulation that's $$ 2(1 + 1.5) = 5 $$ with $d=1.5$.
Whether the relative or the absolute change is more "straightforward" depends on how you want to think about that change. Both ways are useful.