The first thing we are going to do, is draw our triangles (picture 1).
Since both triangles must be similar, the ratios of the lengths of their corresponding sides must be equal.
We can infer from our picture that the segment AB is corresponding to the segment DE and the segment BC is corresponding to the segment EF, so lets find the ratios of those corresponding sides and establish a proportion to find the length of EF:

We know from our problem that

,

, and

, so lets replace those values in our proportion:



We can conclude that the length of EF, such
that ABC ~ DEF, is 10 units.