Carl invested a certain amount of money for one year, some of which earned interest at a simple annual interest rate of 2.2 percent and the rest of which earned interest at a simple annual rate of 2.5 percent. If the amount of money that Carl invested for the year earnead a total of $680 in interest, which of the following could have been the total amount of money that he invested for the year?
I. $24,708
II. $27,896
III. $31,284
A. None
B. II only
C. III only
D. I and II
E. II and III
Let x be the amount invested at 2.2% p.a. and (option given in Q - x) be amount the amount invested at 2.5% p.a.
Option (I):x*\(\frac{2.2}{100}\) + (24708 - x)*\(\frac{2.5}{100}\) = 680
\(\frac{22*x}{1000}\) + (24708 - x)*\(\frac{25}{1000}\) = 680
22x + 24708*25 - 25x = 680000
3x = 24708*25 - 680000
Let this be the standard form we will use for options (II) and (III) too.
3x = 25000*25 - 680000
3x = 625000 - 680000
Since x is coming negative, option (I) is not possible as amount cannot be negative.
Option (II):3x = 27896*25 - 680000 = 28000*\(\frac{100}{4}\) - 680000 = 700000 - 680000
3x = 20000
x = 6667
Also, to double check x is less than 27896, so x is a valid amount. Option (II) is possible.
Option (III):3x = 31284*25 - 680000 = 31200*\(\frac{100}{4}\) - 680000 = 780000 - 680000
3x = 100000
x = 33333
But x is greater than 31284, so x is not a valid amount. Option (III) is not possible.
Only option (II) is allowed.
Answer: B.