Which interprets the set:
{Ø, {Ø}}

Ablock of wood is a cube whose side is x in. long. you cut off a 1-inch thick piece from the entire right side. then you cut off a 3-inch thick piece from the entire top of the remaining shape. the volume of the remaining block is 2,002 in3. what are the dimensions of the original block of wood?

The train station clock runs too fast and gains 5 minutes every 10 days. how many minutes and seconds will it have gained at the end of 9 days?

How is working backward similar to solving an equation?