If you want 15% of 4000 then the question is "what is 15% of 4000?"
"what" means x
"is" means =
"%" means /100
"of" means *
x = 15/100 * 4000
x = 600
Answer: 600 is 15% of 4000.
.
If the question is "what is left if you take 15% from 4000?" then:
Whatever you start with is 100% of itself. If you take 15% away, then you have (100-15)% = 85% left.
Asking "what's left if you take 15% away" is the same thing as asking "what is 85% of"
What is 85% of 4000?
x = 85/100 * 4000
x = 3400
Answer: If you take 15% away from 4000, you have 3400 left.