안녕하세요?

맞아요. decreased by(~만큼 줄어들다) 과 decreased to(~로 줄어들다)를 조심해야해요.

x is decreased by 30% 라면  x - 0.3x = x(1 - 0.3) =  x0.7이 되겠죠

그럼 결국에는 decreased to 70%가 되는거나 마찬가지가 됩니다. 

0.7이 곱해져있으면 decreased by 30% = decreased to 70%를 뜻하게 됩니다.

y = 4(0.7)^x는 output is multiplied by 0.7 when x increases by 1 이라서 

output is decreased by 30% when x increases by 1 가 되겠습니다^^