Find the principal amount

The ending balance on an investment is $1,620.24. If the principal was invested at 8% for nine years, what was the principal?

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interest = principal x rate x time
i = prt

but you are looking at the entire investment

Ending Amount = principal + interest
A = p + prt

1,620.24 = p + p(0.08)(9) {substituted 0.08 for r and 9 for t}
​1620.24 = p + 0.72p {multiplied 0.08 by 9}
1620.24 = 1.72p {combined like terms}
​p = 942 {divided each side by 1.72}

​the principal was $942

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