Hello!
The equation representing the statement is the third one, x=4(x-15). In fact, Juan’s age is x, his age 15 years ago is (x-15) and it is said that his age now is 4 times his age 15 years ago, so x=4(x-15).
If my answer is not clear enough or you’d like further explanation please let me know. Also, English is not my first language, so I’m sorry for any mistakes.
