Answer:
$20
Step-by-step explanation:
Day 1 = $40 ring
Day 2 = you gain $50 meaning you got $10 compensation back
day 3 = $60 you lose your $50 dollars and an added $10
day 4 = you again gain $10 more than what you bought it for adding a $10 compensation
so day one you lost $40
day 2 you gained $50 so saying your bank account is at $0 right now it is at $50
day 3 you loose $60 meaning your bank account is now at -$10
day 4 you gain $70 meaning you bank account is now at $60
you started with $40 and now you are at $60 so you gained $20