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What percentage of the earth's water is available for use by living things?

When your working with the passage-based reading questions on the psat do all of the following except. a)make sure you read each question throughly. b) try to answer each question before you look at the answers. c) plan to spend most of your time reading the passage. d) make sure your answer choice is the most correct number me

How do you work out 21.3-8= show your working. plz