A trapezium-shaped plot has an area of 630 square metres. The perpendicular distance between its two parallel sides is 21 m and one of those parallel sides is 28 m long. How long is the other parallel side, in metres?

From area = $(1/2)(a+b)h$ we get $a+b=2\times 630/21=60$, so $b=60-28=32$ m. The trap 60 quotes the SUM of the two parallel sides instead of the missing one; 2 forgets the factor 2 that undoes the 1/2, giving 630/21 = 30 and then 30 - 28; 88 = 60 + 28 adds where it should subtract, treating 60 as the difference of the parallel sides rather than their sum.