Answer: 50 years.
Step-by-step explanation:
We can use the formula for simple interest to solve this problem:
I = P * r * t
where I is the interest earned, P is the principal amount, r is the interest rate as a decimal, and t is the time in years.
We can rearrange this formula to solve for t:
t = I / (P * r)
Substituting the given values, we get:
t = (500 - 312.5) / (312.5 * 0.04)
t = 625 / 12.5
t = 50
Therefore, it will take 50 years for an investment of Gh312.50 at 4% simple interest to amount to Gh500.00.