Explanation:
it says here "college".
well ...
X + X + X + Y = 4.2
3X + Y = 4.2
X + X + Y = 3.0
2X + Y = 3.0
what is the difference ?
one X (from 3X to 2X) on the left, and 1.2 (from 4.2 to 3.0) on the right.
so, X = 1.2
therefore,
X + Y = 1.8
why ?
because another X (= 1.2) was removed.
and 3.0 - 1.2 = 1.8