Add the first 12 terms of this sequence:
15, 45, 135, 405, 1215, ...

Step-by-step explanation:

a₁ = 15

a₂/a₁ = 45/15 = 3

a₃/a₂ = 135/45 = 3

...

It is a geometric sequence with a common ratio r=3.

Sum of first 12 terms = a₁·(1-r¹²)/(1-r)

= 15·(1-3¹²)/(1-3)

= 15·(-531,441)/(-2)

= 3,985,800

b. 1.25 hr

step-by-step explanation:

if you have a constant speed of 60 mph, then you have a ratio between 60 mi. to 1 hr. dividing both sides by 60 we get the ratio

1 mi. to

then mult. both sides by 75 we get the ratio

75 mi. to

simplifying we get

75 mi. to

or 1.25 hr.