For similar triangles, we can relate them through ratio and proportion. The ratio of ΔABC to ΔDEF is 5/2. We equate this to the measure of AB to the measure of DE, which is 30/x. Calculate for x to find the length of DE.
5/2 = 30/x
5x = 60
x = 60/5
x = 12 mm
Thus, side DE has a length of 12 mm.
