Weighted Average Problem - Best way to approach?

let's cosider customers with plan "A" as a and customers with plan "B" as b.
now,Plan A costs $75 per month per customer.So,its effect is to reduce avarage by $25 per customer.Plan B costs $175 per month per customer. So,its effect is to increase avarage by $75 per customer.
If we add all these deviations,it would add up to zero.

-25a+75b=0
a=3b
[spoiler]percent of the Company's revenue comes from customers with Plan A={(Revenue from plan A)/Total revenue}*100
= 75 a/(75a+175b)*100
= (225b/400 b)*100
=56.25 %
Option D[/spoiler]

Not sure if this is the best way to do it, but:

A = Customers Plan A
B = Customers Plan B

75A + 175B/ A + B = 100

Solving for B we see that B = 1/3A, or 3 times as many A customers than B.

Plugging in 1 customer for B and 3 for A (1:3) we get:

75(3) + 175(1) / 3+1 = 100 = TRUE so:

Total revenue = 75(3) + 175(1) = 225 + 175 = 400

A's portion of 400 = 75 (3) = 225

225 / 400 = 56.25% D

IMOD