A plot shaped like a trapezium has an area of 480 square metres, and its two parallel sides measure 30 m and 34 m. What is the perpendicular distance between the parallel sides, in metres?

From area = $(1/2)(a+b)h$ we get $h=2x$ area/$(a+b)=2\times 480/64=960/64=15$ m. The trap 7.5 divides 480 by 64 and never restores the factor 2 that the 1/2 removed; 240 uses the DIFFERENCE 34 - 30 = 4 in the denominator instead of the sum; 16 = 480/30 treats the plot as a rectangle on the 30 m side.