Question

a sum of money becomes double in 20 years at simple interest. in how many years will it be treble

Answer 1

$\frac{pnr}{100} = si$
by given condition
$\frac{p \times 20 \times r}{100} = 2p \\ r = 10$
Now if it triples
$\frac{pn \times 10}{100} = 3p$
N=30 years