Directly proportional

[joannabanana]

This is from OG, #163

In a certain business, production index p is directly proportional to efficiency index e, which is in turn directly proportional to investment index i. What is p if i=70?
(1) e=0.5 whenever i=60
(2) p=2.0 whenever i=50

I'm unclear on what "directly proportional" means and how to handle it.

[shovan85]

IMO B

Directly proportional means take a constant.
p is directly proportional to e => p = k*e (k is constant)
e is directly proportional to i => e = k' * i (k' constant)

(1) k' = 1/120 but no idea about p (NS)
(2) p = k * k' * i
but k*K' itself will be a constant as multiplication of two constants.
so we can say p = N*i (N new const)
now we can get the value of p for i = 70

[joannabanana]

I'm still confused about this! Can anyone else chime in with a more detailed explanation?