Question 824274
Merry had $20,000to invest. She invested part of this money in bonds paying 10% annual simple interest and the rest of the money in a saving account giving 5%annual interest. At the end of the year she received $1,800 as extra income How much money did merry place in the savings account? 
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Quantity::   t + f = 20,000 dollars
Interest:: 10t +5f = 180000 dollars
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Modify for elimination:
10t + 10f = 200000
10t +  5f = 180000
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Subtract and solve for "f":
5f = 20,000
t = $4000 (amt. invested at 10%)
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t + f = 20,000
t = $16,000 (amt. invested at 10%)
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Cheers,
Stan H.
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