Let:
x = Chemistry textbooks sold
y = Physics textbooks sold
The textbook store sold a combined total of 260, so:
The number of chemistry books sold was three times the number of physics textbooks sold, so:
Let:
Replace (2) into (1):
Replace the value of y into (2):
Step-by-step explanation:
First, let's distribute the 
variable to each term in the first parenthesis:
Next, let's distribute the variable to each term in the second parenthesis:
Next, let's group like terms together:
Finally, let's add or subtract like terms to get the result:
<h3>
Answer: y = 14</h3>
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Explanation:
The two angles involving x add up to 180 degrees. This is because they are same side interior angles (and because the horizontal lines are parallel)
So,
(angle BCF) + (angle EFC) = 180
(5x - 66) + (2x + 50) = 180
5x - 66 + 2x + 50 = 180
7x - 16 = 180
7x - 16+16 = 180+16 ... add 16 to both sides
7x = 196
7x/7 = 196/7 .... divide both sides by 7
x = 28
This will be used to help find y
Notice how the angles BCF and ACD are vertical angles, therefore they are congruent or the same measure
angle ACD = angle BCF
9y - 52 = 5x - 66
9y - 52 = 5*28 - 66 ..... replace x with 28 (since x = 28)
9y - 52 = 140 - 66
9y - 52 = 74
9y - 52+52 = 74+52 .... add 52 to both sides
9y = 126
9y/9 = 126/9 .... divide both sides by 9
y = 14
The answer to your question is 42
