Concrete blocks of length 14/25 m were laid end to end along the four sides of a rectangular lawn 28 m long and 21 m wide. How many blocks were required to complete one layer?

Original speed of X, $S_X = 60 \text{ km/h}$.
Increased speed of X, $S'_X = 60 + 20 = 80 \text{ km/h}$.
Speed of Y, $S_Y = 80 \text{ km/h}$.
Time, $t = 3 \text{ hours}$.

Since they are traveling in opposite directions, their speeds add up.
New relative speed, $S'_{relative} = S'_X + S_Y$

New distance apart $= S'_{relative} \times t$

The original distance was 420 km. The new distance is 480 km.
The effect is that the distance between X and Y will increase by $480 - 420 = 60 \text{ km}$.

Length of lawn, $L = 28 \text{ m}$.
Width of lawn, $W = 21 \text{ m}$.
Perimeter, $P = 2(L + W)$

Length of one block $= \frac{14}{25} \text{ m}$.

Number of blocks $= \frac{\text{Perimeter}}{\text{Length of one block}}$

175 blocks were required to complete one layer.