Slope intercept form word problems

Question

y = mx + b

Phil started a checking account with $50. He only uses his debit card to buy coffee on the way to work each day. If he spends $2 on each coffee, write an equation to show how much he has left after 15 days.

Answer 1

Let amount of money left = y = 50 - 2d, where d is number of days because he buys just one coffee per day. Then y(15) = 50 - 2(15) = 20 dollars.