BTGmoderatorDC wrote:Robin split a total of $24,000 between 2 investments, X and Y. If investment Y earns 7% simple annual interest, how much of that total did Robin put into investment Y?
(1) Each investment earns the same dollar amount of the interest annually.
(2) Investment X earns 5 percent simple annual interest.
Source: GMAT Prep
\[\left. \begin{gathered}
X:\,\,\,24 \cdot k\,\,\,\,\,\,\,;\,\,\,\,\,\,\frac{i}{{100}}\left( {24k} \right)\,\,\,\,\,{\text{annual}}\,\,{\text{simple}}\,\,{\text{interest}}\,\,\left( * \right) \hfill \\
Y:\,\,\,24 \cdot \left( {1 - k} \right)\,\,\,\,\,;\,\,\,\,\,\,\frac{7}{{100}}\left[ {24\left( {1 - k} \right)} \right]\,\,\,\,{\text{annual}}\,\,{\text{simple}}\,\,{\text{interest}}\,\,\, \hfill \\
\end{gathered} \right\}\,\,\,\,\,\,\,\,\left[ {{\text{unit}}:\,\,\,\$ \,\,{{10}^3}} \right]\]
\[\left( * \right)\,\,{\text{just}}\,\,{\text{a}}\,\,{\text{initial}}\,\,{\text{assumption}}\,\]
\[? = 24\left( {1 - k} \right)\,\,\,\,\,\,\,\, \Leftrightarrow \,\,\,\,\,\,\,\,\,\,\,\boxed{? = k}\,\,\,\,\,\,\left( {0 < k < 1} \right)\,\]
\[\left( 1 \right)\,\,\,\frac{i}{{100}}\left( {24k} \right) = \frac{7}{{100}}\left[ {24\left( {1 - k} \right)} \right]\,\,\,\,\,\, \Rightarrow \,\,\,\,\,ik = 7\left( {1 - k} \right)\,\,\,\,\,\left( {**} \right)\]
\[\left\{ \begin{gathered}
\,{\text{Take}}\,\,i = 7\,\,\,\mathop \Rightarrow \limits^{\left( {**} \right)} \,\,\,? = k = 0.5\,\,\,\, \hfill \\
\,{\text{Take}}\,\,i = 0.0001\,\,\,\left( { \cong 0} \right)\,\,\,\mathop \Rightarrow \limits^{\left( {**} \right)} \,\,\,? = k \cong 1\,\,\,\, \hfill \\
\end{gathered} \right.\]
$$\left( 2 \right)\,\,i = 5\,\,\,\left( {{\rm{trivial}}\,\,{\rm{bifurcation}}} \right)\,\,\,\left\{ \matrix{
\,{\rm{Take}}\,\,k = 0.5\,\,\,\, \hfill \cr
\,{\rm{Take}}\,\,k = 0.6\,\,\,\, \hfill \cr} \right.\,\,$$
$$\left( {1 + 2} \right)\,\,\,\left\{ \matrix{
\,\,\left( {**} \right)\,\,\,ik = 7\left( {1 - k} \right)\,\,\,\, \hfill \cr
\,\,\,i = 5 \hfill \cr} \right.\,\,\, \Rightarrow \,\,\,\,k\,\,{\rm{unique}}\,\,\, \Rightarrow \,\,\,\,{\rm{SUFF}}.\,\,\,\,\,$$
This solution follows the notations and rationale taught in the GMATH method.
Regards,
Fabio.
