Answer:
17.5
Step-by-step explanation:
as the triangles are similar, when oriented in the same direction they have the same angles, and the lengths of all sides of DEF are the lengths of the sides of ABC but multiplied by the same scaling factor f for all sides.
so, we see that
A ~ D
B ~ E
C ~ F
and therefore
AB ~ DE
BC ~ EF
CA ~ FD
that means
DE = AB × f
EF = BC × f
FD = CA × f
we know DE and AB.
so,
4 = 10 × f
f = 4/10 = 2/5
and now we know
EF = 7 = BC × f
BC = 7/f = (7/1) / (2/5) = (5×7)/(2×1) = 35/2 = 17.5
