For doing a certain job, boy is offered ₹ 5 on the

For doing a certain job, boy is offered ₹ 5 on the first day, ₹ 15 on the second day, ₹ 45 on the third day and so no. How much will the boy earn at the end of seven days ?

This is a GP with a = 5 , r = 15/5 = 3, n = 7
S_n = a(rⁿ - 1 )/(r - 1)
⇒ S₁₅ = 5(3⁷ - 1)/(3 - 1)
= 5/2(3⁷ - 1)
= 5/2 (2187 - 1)
= ₹ 5465