math help
posted by Peter .
1. a number increased by 21 is 28. find the number
2. a number decreased by 12 is equal to 30. find the number.
3. 8 less than a number is 15. fid the number

See my previous answer to this question.
http://www.jiskha.com/display.cgi?id=1234058590 
can you help me solve this answer and explain it to me.
twice a number plus 7 equal 9 
twice a number plus 7 equals 9
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