Question 800118
let there be x friends initially

6 joined them
so total = x+6

Initially each had to pay 270/x pesos

Later their contribution became 270/(x+6)

270/x - 270/(x+6) = 12

270(x+6)-270x = 12x(x+6)

270x+1620-270x=12x^2+72x

12x^2+72x-1620=0

/12
x^2+6x-135=0

x^2+15x-9x-135=0

x(x+15)-9(x+15)=0
(x-9)(x+15)=0

x=9 OR -15

there were 9 friends initially