Answer:

Step-by-step explanation:
At the start of the day there are g pounds of gravel in the quarry.
Then it says that there were 300 pounds added to the mound, so at this point the pounds of gravel in the mound are now g+300.
Then two orders of 700 pounds are sold and removed from the mound. That means they removed 700 pounds from the mound 2 times. Therefore at this point the pounds of gravel in the mound are now g+300-700-700.
Then it says that at this point, the amount of pounds in the mound are 1500. Hence we get

To solve for g we just add 700 two times to both sides of the equation, and subtract 300 from both sides of the equation, getting:

And so 
The 3rd one
This is because in the graph it’s looking for numbers 70-79 but only with 2 numbers and the 3rd is the only one that fits in the description.
We have the equation of the line as follows:
y = 0.5x + 79
We want to calculate the melting point (the y value) of DNA with 50% gc content (the x value).
The gc content represents the value of x in the equation, so to calculate the melting point (y), we will simply substitute the x with 50 in the equation of the line as follows:
melting point = 0.5(50) + 79 = 25 + 79 = 104 degrees