Trevor solved the system of equations below. what mistake did he make in his work?

2x + y = 5
x − 2y = 10

y = 5 − 2x
x − 2(5 − 2x) = 10
x − 10 + 4x = 10
5x − 10 = 10
5x = 0
x = 0

2(0) + y = 5
y = 5

he should have substituted 5 + 2x
he combined like terms incorrectly, it should have been 4x instead of 5x
he subtracted 10 from the right side instead of adding 10 to the right side
he made no mistake

Can someone tell me if this is perpendicular? !

Acertain volume of water contains 100000 hydrogen atoms and 50,000 oxygen atoms how many hydrogen atoms are in a volume of water containing 4 million oxygen atoms

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