One kilogram of a certain coffee brand consists of x kilograms of Type I coffee and y kilograms of Type II coffee. The cost of the brand is C dollars per kilogram, where C = 6.5x + 8.5y. Is x < 0.8?

1) y >= 0.15

2) C >= 7.30

x+y=1
Statement 1 implies that x could be less, equal or more than 0.8; not sufficient.

Statement 2 certainly is.

The correct answer is B

x + y = 1

If y>=0.15, then try some:
If y=0.15, x = 0.85 in which case, no, x is not less than 0.8.
If y=0.3, then x=0.7, in which case, yes, x is less than 0.8.
Contradictory answers = indufficient.

y = 1-x
If C >=7.30, then try some:
If C=7.30, then 7.3 = 6.5x + 8.5(1-x)
7.3 = 6.5x + 8.5 - 8.5x
-1.2 = -2x
1.2/2 = x
x = 0.6, so yes, x < 0.8.

What happens if I increase C? I can either try a larger number or use logic to figure out if x will go up or down - totally up to you on the real thing.

If x+y=1, and I need to create a larger C, I have to make x smaller and y larger (because I multiply y by 8.5 in the formula while only multiplying x by 6.5). So, x will decreas as C increases. As a result, I can always say that x < 0.8. Sufficient.