The membership of a music club costs fifteen dollars per month, as shown in the table. Write an equation for the total cost, 𝑑, of a music club membership for a duration of 𝑡 months.
And the table shows that after one month, it costs fifteen dollars. After two months, it’s thirty dollars, three months is forty-five dollars, and four months is sixty dollars. Now a couple of things to look at before we move on, every time we increase the duration by one month, we can see that the price that we pay in total goes up by fifteen dollars. And that’s consistent throughout the table: every extra month we add to a membership costs an extra fifteen dollars.
But we can also see that if we take the number of months and multiply it by fifteen, it tells us what the cost is. So two months multiplied by fifteen, two times fifteen is thirty, three months times fifteen is forty-five, and four months times fifteen is sixty.
So the total cost is the number of months times fifteen or fifteen times the number of months. Now the cost, we said we’re gonna represent with the letter 𝑑 and the number of months we’re gonna represent with the letter 𝑡. So we can write this as 𝑑 equals 𝑡 times fifteen. But rather writing 𝑡 times fifteen, we normally put the number first. So the formula for the total cost is 𝑑 is equal to fifteen 𝑡. So let’s make our answer stand out nice and clearly on the page.