the longer side / the shorter side must be the same for rectangles to be similar.
For A: 105 / 80 = 1.32
For B: 120 / 90 = 1.33
For C: 100 / 75 = 1.47
So... only A and B are similar.
31.+ addition
32. - subtraction
The above questions answer is 8 and 14
Check the picture below.
now, keep in mind that ship B is going at 20kph, thus from noon to 4pm, is 4 hours, so it has travelled by then 20 * 4 or 80 kilometers, thus b = 80.
whilst the ship B is moving north, the distance "a" is not really changing, and thus is a constant, that matters because the derivative of a constant is 0.