A sum of Rs.10 is lent by a child to his friend to be returned in 11 monthly instalments of Rs.1 each, the interest being simple. The rate of interest is:

$21\frac{9}{11}\%$

Interest = 1 Rs

Interest = \(\frac{Principal ×Rate×Time}{100}\)

Total Interest for every month=

1 = \(\frac{10×R×1}{100×12}\)+\(\frac{9×R×1}{100×12}\)------\(\frac{1×R×1}{100×12}\)

1 = \(\frac{(10+9+8-----1)×R×1}{100×12}\)

(Sum = 10+9+8-----1=\(\frac{10×11}{2}\)=55)

1=\(\frac{55×R×1}{1200}\)

R=\(\frac{240}{11}\) OR 21 \(\frac{9}{11}\)%