For simple interest, it is 20%.
A = P(1 + r*t) (formula for an amount at simple interest rate r over period t)
2P = P(1 + r*t) (we want to double our money)
2 = 1 + r*t (the amount of money is irrelevant. We can divide it out.)
1 = r*t (subtract 1)
1/t = r (divide by t to find r)
1/5 = r (use the given value for t)
1/5*100% = 20% = r
A = P(1 + r*t) (formula for an amount at simple interest rate r over period t)
2P = P(1 + r*t) (we want to double our money)
2 = 1 + r*t (the amount of money is irrelevant. We can divide it out.)
1 = r*t (subtract 1)
1/t = r (divide by t to find r)
1/5 = r (use the given value for t)
1/5*100% = 20% = r