Math
posted by Crystal .
A clothing store ordered 300 sweaters and sold 20 of them at $100 the first week. The second week, the selling price was lowered by $10, and 40 sweaters were sold. In the third week. The selling price was lowered by another $10, and 60 sweated were sold. If the pattern continues,
A) how many weeks did it take to sell all sweaters?
B) what was the selling price in the final week?
For a) I started off by writing it out:
Number of sweaters each week: 300, 280, 240, 180
I got stuck because they don't reduce by a constant common ratio or common difference. How would you figure out this type of question?
Thanks in advance!

Week 1: 20 sweaters at $100 = $2000
Week 2: 40 sweaters at $90 = $3600
Week 3: 60 sweaters at $80 = $4800
Week 4: 80 sweaters at $70 = $5600
Week 5: 100 sweaters at $60 = $6000
100+80+60+40+20 = 300 sweaters sold in 5 weeks
A) Took 5 weeks
B) $60 a sweater in the final week 
Ohhhh. Thank you ! I was going about it backwards!

Thanks for the answer