ans: The overall annual rate of interest is 11.89%.

Basic: সরল সুদের ক্ষেত্রে 1 = $\frac{Prt}{100}$
Here, for Tk. 10,000 interset, $I_{1}$ = 10,000 x 1 x $\frac{12.50}{100}$ = 1,250 Tk.
For Tk. 15,000 interest of 6 month = 15,000 x $\frac{1}{2}$x $\frac{12}{100}$ = 900 Tk.
For Tk. 15,000 Interest for 1 year would be, $I_{1}$ = 900 x 2 = 1,800 Tk.
Now, For Tk. 20,000 interest of 3 month = 20,000 x $\frac{3}{12}$ x $\frac{11.50}{100}$ = 575 TK.
For Tk. 20,000, Interest for 1 year would be, $I_{3}$ = 575 x 4 = 2,300 Tk.
Total investment = 10,000 + 15,000 + 20,000 = 45,000 Tk.
Total interest for 1 year = $I_{1}$ + $I_{2}$ + $I_{3}$ = 1,250 + 1,800 + 2,300 = 5’350 Tk.
After investing Tk. 45,000 in one year we have = $\frac{5350\times 100}{45000}$ = 11.89 Tk