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So the unknown is how much he invested at 8%, so that will be the variable, x.
We know he invested $100 more at 7%, so he invested x+100 at 7%.
Simple interest is not compounded, so we know that since it says "annual" interest, the time period is one year, so he will make exactly 7% and 8% of the amounts. Therefor, we can use the following formula:
Make sure you convert 8% to .08 and 7% to .07. Percent means "per 100", so 7% is 7/100, or .07.
distribute the .07 to get:
combine like terms
subtract 7 from both sides
divide both sides by .15
this means that 500 was invested at 8% and (x+100 so 500+100) 600 was invested at 7%.