AAPL wrote:Magoosh

In 2004, 2,400 condos sold, 15% of the total housing units sold that year. How many units sold in 2004 were not condos?

A. 16,000
B. 13,600
C. 12,400
D. 11, 200
E. 8,600

Of the units sold, non-condos constitute 85% = 85/100 = 17/20.
Implication:
Of every 20 units sold, 17 are non-condos, with the result that the number of non-condos must be a multiple of 17.

A. 16000
B. 13600
C. 12400
D. 11200
E. 8600

Only the portion in green is a multiple of 17:
136/17 = 8.

The correct answer is B.

AAPL wrote:Magoosh

In 2004, 2,400 condos sold, 15% of the total housing units sold that year. How many units sold in 2004 were not condos?

A. 16,000
B. 13,600
C. 12,400
D. 11, 200
E. 8,600

Let´s use the "backbone" of our method: connect DATA to FOCUS as soon and as clearly as possible:
\[{\text{DATA}}:\,\,\,\,\,2,400\,\,{\text{condos}}\,\,\,\mathop = \limits^{\left( * \right)} \,\,\,\frac{{15}}{{100}}\,\,\left( {{\text{total}}} \right)\]
\[{\text{FOCUS}}:\,\,\,? = \frac{{85}}{{100}}\left( {{\text{total}}} \right)\]
\[? = \boxed{\frac{{85}}{{15}}} \cdot \underbrace {\frac{{15}}{{100}}\,\,\left( {{\text{total}}} \right)}_{\left( * \right)\,\,\,2400}\,\, = \,\,\,\frac{{17}}{3}\left( {2400} \right) = \underleftrightarrow {100 \cdot \left( {\frac{{17 \cdot 24}}{3}} \right)}\,\, = \,\,13600\]

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

Regards,
Fabio.