(18) i did x-3/16 = 2.5/4
40 = 4x - 12
28 = 4x
x = 7

See the attached image to have a look at how the corresponding sides pair up
The red sides pair up to form the ratio (x-3)/2.5
The blue sides pair up to form the ratio 8/2
The similar figures mean that the two ratios are equal

(x-3)/2.5 = 8/2
2(x-3) = 2.5*8
2x-6 = 20
2x = 26
x = 13

Side note: Solving 40 = 4x-12 should lead to 52 = 4x which means x = 13. You subtracted 12 from both sides instead of adding 12 to both sides.

2v

Step-by-step explanation:

Let the distance be d and average speed be v.

Half of the distance the speed was 2/3v

Let the speed be xv in the second half of the distance.

Then time is:

d/v = d/2 : (2/3v) + d/2 : (xv)d/v = 3/4(d/v) + (1/2x)(d/v)1 = 3/4 + 1/2x1/2x = 1 - 3/4 1/2x = 1/42x = 4x = 2

The rest of the road car has to ride with speed of 2v