At what rate of simple interest will a sum of money amount to $\frac{7}{4}$ of itself in $8$ years and $4$ months?

At some rate of simple interest, A lent $₹6,000$ to B for $2$ years and $₹1,500$ to C for $4$ years and received $₹900$ as interest from both of them together. The rate of interest per annum was?

If a principal becomes twice of it in $10$ years, then the rate of simple interest per annum is

Interest on $₹a$ at the simple interest $10\%$ per annum for $b$ months is:

A sum of $₹10000$ is invested in three schemes of simple interest. The annual interest rates are respectively  $4\%$, $6\%$ and $10\%$. $₹4000$ were invested in the first scheme. If the total interest earned after five years is $₹2800$, then how much money was invested in the third scheme?