The answer would be option B "he was given $50 for his birthday."
Here is how:
Y=
^
Y equals the amount of money in the account.
75$+50$=
75x represents the amount of money he DEPOSITS which is in the question.
So if he deposits 75$ a week (which represents X) and he adds 50$ to it which means he has 50$ to begin with which means he got 50$ for his birthday.
Also....
Because 75x already represents X therefore it wouldn't be options A, C or D.
Hope this helps!
