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Home / arithmetic / A sum of interest Tk. 750 at the interest of 8% and another sum of Tk. 1250 at 6% were invested. What is the average percent rate of interest of the total sum of interest?

A sum of interest Tk. 750 at the interest of 8% and another sum of Tk. 1250 at 6% were invested. What is the average percent rate of interest of the total sum of interest?

Solution:
For 1st one, the rate of interest = 8%
Principle = Tk. 750
For second one,
The rate of interest = 6%
Principle = Tk. 1250
Interest of Tk. 100 in 1 year = Tk. 8
∴ ”            ”  ”         1 ”     1 ”      = (8÷100)
∴ ”             ”  ”      750 ”   1 ”     = (8×750)÷100
= Tk. 60
Again, interest of Tk. 100 in 1 year = Tk. 6
∴                ”      ”     ”         1″     1 ”      = (8÷100)
∴                ”       ”   ”      1250 ”  1 ”      = (8×1250)÷100
= Tk. 75

∴ Total Principle= Tk.(750 + 1250)
= Tk. 2000
And total interest = Tk. (60+75)
= Tk. 135
So,
Interest of Tk. 2000 in 1 year = Tk. 135
∴  ”          ”   ”            1  ” 1     ”     = (135÷200)
∴  ”         ”    ”            100 ” 1   ” = (135×100)÷200
= Tk. 6.75
Answer: 6.75%

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