D.
This is because it is a question that involves doing actual math rather than just asking and getting an answer.
-5x = - 7 because -6x plus x is -5x and use elimination to get rid of the -3y so you end up with -5x = -7
Answer: 4
Step-by-step explanation:
60 = (10 + 20 + 30 + 40 + x)/5
300 = 100 + x
x = 200
Answer: 200
Set it up like a regular equation. I used:

As you can see, however, I inserted the "less than" symbol rather than the equal sign because values greater than a certain value can also equal "x". Solve it like a normal equation and you will arrive at:
<u>
</u>

This means that, as long as x is greater than 45, "6x" will be greater than "3x+135". Thus, if you bring 46 students, the Science Center will cost less.