Answer:
Choice C) 300
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Work Shown:
Solve the first equation for y. Subtract x from both sides.
x+y = 525
x+y-x = 525-x
y = 525-x ... we'll use this later
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Plug that into the second equation. We replace every 'y' with (525-x). The parenthesis are important.
3x + 7y = 2775
3x + 7( y ) = 2775
3x + 7( 525-x ) = 2775
3x + 7(525) + 7(-x) = 2775 ... distribute
3x + 3675 - 7x = 2775
-4x + 3675 = 2775
-4x + 3675 - 3675 = 2775 - 3675 ... subtract 3675 from both sides
-4x = -900
-4x/(-4) = -900/(-4) ... divide both sides by -4
x = 225
There are 225 children.
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Plug that result into the equation in which we solved for y earlier.
y = 525-x
y = 525-225 ... replace x with 225
y = 300
There are 300 adults.