Shirley Invested A Total Of 12000 In Two Certificates Of Deposit One Pays 6 Percent Interest Ant The Other Pays 7 Percent Interest Her Total Interest At The End Of One Year Is 790 How Much Is Invested?

Suppose x is the amount invested at 7%. The amount of interest earned totals $790.
  X*7% + ($12,000 - x)*6% = $790
  x*(7% - 6%) + $12,000*6% = $790
  x*1% = $790 - $12,000*6%
  x = $(790 - 720)/1%
  x = $70/.01
  x = $7000

Shirley invested $7000 at 7% and $5000 at 6%.