Pls help me understand the logic of these 2 DS problems

First question:

Suppose there 100 projects.
Statement 1 tells us that 25 projects have 4 or MORE people working on them.
These have to be the upper-end projects, since 4 or more people are working on them. So we know that the other 75 projects have fewer than 4 people working on them, but we can't say exactly how many.

Statement 2 tells us that the 35 lower-end projects have 2 or LESS people working on them. But it doesn't tell us anything about the middle.

Combining together,

---2 or less (35)-----40 has to be 3--------4 or more (25)

So there are 40 projects in the middle that HAVE to have exactly 3 people working on them.
So the answer is C

We know that x+y =1
6.5x + 8.5y = C

Statement1 tells us that y>0.15, then if x+y=1, then x=1-y
And x<0.85, so we can't tell for sure if x is less than 0.8. Can be 0.1 or 0.84
So, insuff

Statement 2 tells us that C >=7.30
Then,
6.5x + 8.5y>= 7.3
y=1-x--->6.5x+8.5(1-x)>=7.3------>6.5x+8.5-8.5x>=7.3
2x<1.2--->x<0.6 and is obviously less than 0.8

Answer is B

I think that the catch in the second problem is to realize that x+y=1, then you'll be able to eliminate one extra unknown.