Answer
(C) 15
Explanation
The given equation that represents the number of bags of sand (x) and bags of mulch (y) that can be bought with $105 is

If no bags of sand are bought, therefore x = 0
Then the number of bags of mulch that can be bought with $105 will be
-3x+y=10 is the answer. If you plug the coordinates in to the respective spots (-2 for x and 4 for y), you will get an equal true statement.
So we know that there are 1000 mL in 1 L. We can set up an equation that allows us to solve for how many bottles are needed in order to empty the 30 L tank:

This equation then allows us to cancel out the units of mL and L, leaving us with how many bottles are needed per tank. Then we multiply straight across to give us:

The question asks us to round up to the next whole number, so
it would take 86 bottles to empty one 30 L tank.
the first two are correct, but the answer for c should be 125