Let's sample some data to recognize the trend: at the beginning (t=0), mr Loung has made no money. So,
[tex] t = 0 \to d = 0 = 15 \cdot 0 [/tex]
After the first hour (t=1), he has made 15 dollars:
[tex] t = 1 \to d = 15 = 15 \cdot 1[/tex]
After two hours (t=2), he has made another 15 dollars:
[tex] t = 2 \to d = 15+15 = 30 = 15\cdot 2 [/tex]
After three hours (t=3), he has made another 15 dollars:
[tex] t = 3 \to d = 30+15 = 45 = 15\cdot 3 [/tex]
Can you see the pattern? After a certain number of hours, the number of dollars earned is always 15 times that number of hours. So, the general equation is
[tex] d = 15t [/tex]
