Kristi gets $10 a week from her parents if she washes the dishes after dinner every night. Kristi has saved up $30 over the last few weeks from her allowance. She wants to continue saving her allowance until she has enough money to buy the $70 bike she saw at the store last week. Assuming Kristi receives her allowance every week, write an equation that shows how many more weeks Kristi has to save her allowance to buy the bike, where w is the number of weeks.