Hi mttorii,

My solution is:

First, rewrite the problem clause in an equation.
0,1x+0,02y=0,05(x+y)

Than
(1) y=10
substitute y in the eqation
0,1x+0,02*10=0,05(x+10)
This is one equation with one unknown. That is, we can solve for x.
SUFFICIENT

(2) z=16
Because the mixture contains gravel x and gravel Y ONLY, z=x+y.
So we have the following system of two equations with two unknowns
0,1x+0,02y=0,05(x+y) and
x+y=16
That is, we can solve this system for x and y.
SUFFICIENT

In both cases we have that x=6 and y=10.

The answer is D, isn't it?

Best!

Hi Ed09

Yes, D is the answer. Very didatic!
Thank you.

There is another way to look over this problem

when have given X tons = 10% of G
Y tons = 2% of G
Z tons = 5% of G

In the first statement we have given the value of Y. plugging in the value of Y we can find the G therefore after getting the value of G we can find the value of X

Same logic applies to statement II

So both the equation is sufficient . hence OA is 'D'