The ages of people signed up for piano lessons are listed.

11, 16, 15, 9, 19, 12, 13, 13, 11, 11, 18, 17

what is the mode of the ages?

a.
12

b.
15

c.
13

d.
11

In the figure below, ∠dec ≅ ∠dce, ∠b ≅ ∠f, and segment df is congruent to segment bd. point c is the point of intersection between segment ag and segment bd, while point e is the point of intersection between segment ag and segment df. the figure shows a polygon comprised of three triangles, abc, dec, and gfe. prove δabc ≅ δgfe.