Question 853211
Let X be the number of days.
Start with 50 and add 4 for X days.
50 + 4X = amount saved
.
You can prove this by plugging in some values. 
Let's try a week.
If you save $4 a day for a week, you'll have $28 plus the initial $50.
That would be:
50 + (4*7) = 78
50 + 28 = 78
Success!