Answer:
B. 10
Step-by-step explanation:
First, we need to recognize that the two triangles are similar (the triangle with a leg value of 21 and the triangle with a leg value of 4 and hypotenuse x)
Let's first label the leg of the second triangle as y.
Then, we can set up this equation

Cross multiplying gives us
84=y^2
So, y=sqrt(84) or 2 sqrt(21).
Now, using the pythagorean theorem, we get
4^2+2sqrt(21)^2=x^2
100=x^2
x=10
10 x 40 = 400 so the answer is 400 pieces of chalk.
X= cost per cherry pie
y= cost per pumpkin pie
NICOLE
1x + 9y= $60
LISA
11x + 4y= $90
STEP 1
multiply Nicole's equation by -11
-11(1x + 9y)= -11($60)
multiply -11 by all terms
(-11 * x) + (-11 * 9y)= (-11 * 60)
-11x - 99y= -660
STEP 2
add Nicole's new equation from step 1 to Lisa's equation to solve for y (using the elimination method)
-11x - 99y= -660
11x + 4y= 90
the x terms "cancel out"
-95y= -570
divide both sides by -95
y= $6 per pumpkin pie
STEP 3
substitute y value into either original equation to solve for x
x + 9y= $60
x + 9(6)= 60
x + 54= 60
subtract 54 from both sides
x= $6 per cherry pie
CHECK
11x + 4y= $90
11(6) + 4(6)= 90
66 + 24= 90
90= 90
ANSWER: Each cherry pie costs $6 and each pumpkin pie costs $6.
Hope this helps! :)