Answer: The wingspan is 36 m.
Using a proportion to solve this, we write the scale factor as a ratio first: 19/38, since 19 is the size of the model's length and 38 is the real length. For the second ratio, we have 18 for the size of the model wingspan and x for the real wingspan:
19/38 = 18/x
Cross multiply:
19*x = 38*18
19x = 684
