fskilnik
GMATH practice exercise (Quant Class 16)
If x, y, and z are integers such that 67500 is divisible by (2^x)(3^y)(5^z) and (2^x)(3^y)(5^z) is NOT a multiple of 54, what is the maximum possible value of 3x+2y+z?
(A) 15
(B) 14
(C) 13
(D) 12
(E) less than 12
Very nice,
arosman. Thank you for your contribution (and kudos)!
Our official solution follows:
\(? = \max \left( {3x + 2y + z} \right)\,\,\,\left( * \right)\)
\(x,y,z\,\,\mathop \ge \limits^{\left( * \right)} \,\,0\,\,\,{\rm{ints}}\,\,\,\left( {**} \right)\)
\(67500 = \underleftrightarrow {675 \cdot 100} = 25 \cdot 27 \cdot 4 \cdot 25 = {2^2} \cdot {3^3} \cdot {5^4}\)
\(54 = 2 \cdot 27 = 2 \cdot {3^3}\)
\(\left. \matrix{\\
{\mathop{\rm int}} = {{\,{2^2} \cdot {3^3} \cdot {5^4}\,} \over {{2^x} \cdot {3^y} \cdot {5^z}}}\,\,\,\,\,\mathop \Rightarrow \limits^{\left( {**} \right)} \,\,\,\,\,\left\{ \matrix{\\
\,0 \le x \le 2 \hfill \cr \\
\,0 \le y \le 3 \hfill \cr \\
\,0 \le z \le 4 \hfill \cr} \right. \hfill \cr \\
{\mathop{\rm int}} \ne {{\,{2^x} \cdot {3^y} \cdot {5^z}\,} \over {2 \cdot {3^3}}}\,\,\,\, \Rightarrow \,\,\,\,x = 0\,\,{\rm{or}}\,\,y < 3\,\,\left( {{\rm{or}}\,\,{\rm{both}}} \right)\,\, \hfill \cr} \right\}\,\,\,\,\,\,\mathop \Rightarrow \limits^{\left( * \right)} \,\,\,\,\,{\rm{Take}}\,\,\,\left( {x,y,z} \right) = \left( {2,3 - 1,4} \right)\,\,\,\,\, \Rightarrow \,\,\,\,\,? = 14\)
The correct answer is (B).
We follow the notations and rationale taught in the
GMATH method.
Regards,
Fabio.