First set up your two equations:
x + y = 90
x = 2y - 30
Then substitute what x equals in the second equation into the first equation:
(2y -30) + y = 90
Now solve for y:
3y -30 = 90
3y = 120
y = 40
Then use y = 40 and substitute the value for y into one of your original equations and solve for x. I'll choose the first one, but either one will work.
x+ 40 = 90
x = 50
So your solution is x = 50 and y = 40
To answer this you need to make simple thought experiment. Since the number of sales after time passes cannot go negative (note that rate at which copies are sold can drop over time but total number cannot) that means that you are adding number of sales at first hour and during first hour.
first option cannot be the answer because you are subtracting 100 copies to total number of sold copies which is not logical.
The answer is D) s(x) = 100 + 12(x-1)
after 1 hour (if you replace x with 1 in equation above) you will get 100 copies which matches conditions in the text.
The answer is 2/3 shirt per hour.
The surface of the triangle count formula
A = b*h*1/2
A - area
b - base
c - height
The 1st triangle
A₁ = b₁*h = 6in * 8in *1/2= 24in²
The 2nd triangle
A₂ b₂*h = 10in * 8in*1/2 = 40in²
The total area of the two triangles is 24in² + 40in² = 64 square inches
