We can use the formula for simple interest to solve for t:
I = P * r * t
where I is the interest, P is the principal amount borrowed, r is the interest rate, and t is the time period in years.
In this case, Mary borrowed $5,000 at a 3% simple interest rate, so we can substitute these values into the formula:
$600 = $5,000 * 0.03 * t
Dividing both sides by $5,000 * 0.03, we get:
t = $600 / ($5,000 * 0.03)
t = 4 years
Therefore, the value of t is 4 years.