Answer: 
Step-by-step explanation:
All that is required here is to substitute x = 2x into the right side in the same way that is done for a numeric value.

X - $5 bills
y - $10 bills
x + y = 12
y = 5x
x + (5x) = 12
6x = 12
x = 2
y = 5(2)
y = 10
10 $10 bills and 2 $5 bills
Answer:
B and C
Step-by-step explanation:
The correct option are
B) a cross section of rectangular pyramid perpendicular to the base
C) a cross section of a rectangular prism that is parallel to it's base