Answer: B. 8th day: 100 minutes; 9th day: 110 minutes; 10th day: 120 minutes
Remember:
[tex]a_{n}[/tex] = [tex]n[/tex]th term
[tex]n[/tex] = number of terms
[tex]a_{1}[/tex] = first term
[tex]d[/tex] = common difference
Identify the given information.
[tex]a_{1} = 30\\d = 10[/tex]
Choose which formula to use. In this case, you should use the explicit formula for arithmetic sequences.
[tex]a_{n} = a_{1} + d(n - 1)[/tex]
Substitute in the given values and simplify.
[tex]a_{n} = 30 + 10(n - 1)\\a_{n} = 30 + 10n - 10\\a_{n} = 10n + 20[/tex]
Substitute in 8, 9, and 10 for [tex]n[/tex] to find out how many days Rick would train on the 8th, 9th, and 10th day of training.
[tex]a_{8} = 10(8) + 20 = 80 + 20 = 100\\a_{9} = 10(9) + 20 = 90 + 20 = 110\\a_{10} = 10(10) + 20 = 100 + 20 = 120[/tex]
