Calculate the rate of interest for the following principal, interest, and time periods: a) P=4000, I=800, T=2yrs b) P=3000, I=1350, T=3yrs c) P=10000, I=4000, T=4yrs

$P = \text{Rs } 4000$
$I = \text{Rs } 800$
$T = 2 \text{ yrs}$

The rate of interest is $\boxed{\text{10\%}}$.

$P = \text{Rs } 3000$
$I = \text{Rs } 1350$
$T = 3 \text{ yrs}$

The rate of interest is $\boxed{\text{15\%}}$.

$P = \text{Rs } 10000$
$I = \text{Rs } 4000$
$T = 4 \text{ yrs}$

The rate of interest is $\boxed{\text{10\%}}$.

$P = \text{Rs } 15000$
$I = \text{Rs } 7500$
$T = 5 \text{ yrs}$

The rate of interest is $\boxed{\text{10\%}}$.