At what simple interest rate percent per annum will a sum of money triple in 21 years?

Answer 1

9.52%

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Amount of money triples -> increases by 200% in 21 years,
21x=200% -> x=200%/21= 9.52%

Answer 2

PTR/100 = 2P
TR = 200
R = 200/21 = 9.5238%.