Answer
Less than or equal to 4 days.
Explanation:
Let D represent the number of days for Starlin's trip. We know that Starlin has spent $25 on gas and will spend $100 for each day. So use the equation d=25+100d < 415. (We also know that Starlin plans on spending less than $415 on the trip, so if anything is above $415, we know it is the wrong answer.) Subtract 25 on both sides and then divide by 100; the equation should go like this:
25 + 100d < 415
25 + 100d - 25 < 415 - 25
100d < 390
d < 3.9
It would be wise to round to 4 unless otherwise specified.