Answer:
The principal amount was $500.
Step-by-step explanation:
Simple (non-compound) interest $90 for three years means $30 for 1 year.
Knowing the interest amount for one year and the interest rate 0.06 (6%) we can determine the principal from:
(interest) = (principal) x (interest rate)
$30 = (principal) x 0.06
=> (principal) = $30 / 0.06 = $500