Let total bill would be ₹ P
Each one will to pay = ₹ P/11
10 friends could pay 10 x 60 = ₹ 600
According to question
600 + P/11 + 50 = P
⇒ 650 = P - P/11
⇒ 650 = (11P - P)/11
⇒ 650 x 11 = (11P - P)
⇒ 650 x 11 = 10P
⇒ P = 11 x 65
⇒ P = 715
Amount paid by 11th friend = 715/11 + 50 = ₹ 115