a70 wrote:

If m and n are positive integers and m * n = 40 , what is the value of the sum of m and n?

(1) The number of positive factors of m is twice the number of positive factors of n.

(2) m has exactly 4 different positive factors

\(\left. \begin{gathered}

m,n\,\, \geqslant 1\,\,\,{\text{ints}}\,\, \hfill \\

mn = 40\, \hfill \\

\end{gathered} \right\}\,\,\,\,\, \Rightarrow \,\,\,\,m\,\,{\text{and}}\,\,n\,\,{\text{are}}\,\,{\text{pairs}}\,\,{\text{of}}\,{\text{positive}}\,\,{\text{factors}}\,\,{\text{of}}\,\,40\)

\(? = m + n\)

This is a perfect opportunity to present our "

T diagram" (see image attached), GMATH´s creation to help students find ALL positive factors of a given positive integer.

We all know that 40 = (2^3)*5 has 4*2 = 8 positive factors. The

T technique shows them explicitly and, more than that, in the corresponding pairs (in the four rows)!

We have put the number of positive factors of each positive factor of 40 in parentheses. A quick inspection in each pair of divisors shows us that:

\(\left( 1 \right)\,\,\,\, \Rightarrow \,\,\,\left( {m,n} \right) = \left( {{2^3},5} \right)\,\,\,\,\, \Rightarrow \,\,\,? = 13\,\,\,\,\, \Rightarrow \,\,\,{\text{SUFF}}.\,\,\,\,\,\,\)

\(\left( 2 \right)\,\,\, \Rightarrow \,\,\,\left\{ \begin{gathered}

\,\left( {m,n} \right) = \left( {2 \cdot 5,{2^2}} \right)\,\,\,\,\, \Rightarrow \,\,\,? = 14\,\,\,\,\, \hfill \\

\,\left( {m,n} \right) = \left( {{2^3},5} \right)\,\,\,\,\, \Rightarrow \,\,\,? = 13\,\,\,\, \hfill \\

\end{gathered} \right. \Rightarrow \,\,\,{\text{INSUFF}}.\)

The correct answer is therefore (A).

This solution follows the notations and rationale taught in the GMATH method.

Regards,

Fabio.

Attachments

GMATH_T_diagram_25Set18.gif [ 14.9 KiB | Viewed 238 times ]

_________________

Fabio Skilnik :: http://www.GMATH.net (Math for the GMAT)

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