A sum of money doubles in 14 years. In how many years would it triple if the rate of interest is considered as simple?

It triples in 28 years.

Doubling in 14 years means the interest earned equals the principal in 14 years.

Under simple interest the interest grows at a constant rate.

Tripling needs interest equal to twice the principal.

Twice the interest needs twice the time, 2 x 14 = 28.

So the sum triples in 28 years.