X = amount of seeds.
y = amount of dried fruits.
we know the snack mix contains both, and we know is 10oz, thus 
x + y = 10, whatever ounces "x" and "y" are.
how much is it for "x" ounces if each one costs $1.5?  well is just 1.5*x or 
1.5x.
how much is it for "y" ounces if each one costs $2.5?  well is just 2.5*x or 
2.5x.
the mix contains 10 ounces, each of which costs $2.2 each, how much will it be then for 10 oz?  well, is just 10 * 2.2, or $22.
since the whole 10 oz snack mix costs 22 bucks, then 
1.5x + 2.5y = 22.

how many ounces of dried fruit is there anyway?  well, y = 10 - x.