One day you decide to get stronger by doing pushups. You start by doing 5 pushups on the first day. Then you increase it to 10 on the second day. Then 15 on the next day, and you continue this pattern for a week. Write an equation that relates the number of pushups y to the number of days x during the week
1. x-y=5
2. x+y=5
3. x=5y
4. y=5x