What does perpendicular mean?

Alex took his brother to a birthday party. the party was scheduled to last for 1 and 3/4 hours but they stayed for 4/5 of an hour more. how long did she stay at the party?

Which equation can be used to measure the density of water in a spherical water tank? let w represent the number of water particles within the region.

If each bricks costs and he can only buy one brick how much will it cost him to get the material to put around the outside of his garden?