let x
be the amount of perfume at $15/oz and y be the amount at 35/oz
x + y = 3
rewrite
is as x=3-y
15x + 35y = 63
substitute for x into the second equation
15(3 - y) + 35y = 63
45 - 15y + 35y = 63
20y = 18
y = 20/18
= 0.9 oz
x = 3-0.9= 2.1 oz
2.1 oz of $15/oz perfume