Tiffany is borrowing 1,200 from her parents to buy a new laptop computer. She plans to pay them back at the rate of $70 a month. Rosie is borrowing $900 from her parents to buy a violin. She plans to pay her parents back at the rate of $40 per month

This isn't really a question so I'll just go through all of it.

1200 dollars will take about 17 months to pay off with installments of 70 dollars.

900 dollars will take 22.5 months to pay off with 40 dollar installments.

Tiffany will pay off her debt first.

you need a common difference between consecutive terms to be constant, like 1,2,3,4 or 2,4,6,8, or 1,3,6,9