sn=a 1*(q^n- 1)/(q- 1)

s5=a 1*(q^5- 1)/(q- 1)= 10

s 10=a 1*(q^ 10- 1)/(q- 1)=50

Two types of division:

q^5+ 1=5

q^5=4

a 1*(q^5- 1)/(q- 1)= 10

a 1/(q- 1)= 10/3

s 15=a 1*(q^ 15- 1)/(q- 1)

=a 1*(4^3- 1)/(q- 1)

=63a 1/(q- 1)

=63*( 10/3)

=2 10