Physics question? (suvat) UKCAT

Explain to me how is accelerating from rest to 60mph in 6s is equivalent to moving at 30mph for 6s????

The QR on the UKCAT mock 1 is unbelievably hard!!

If you draw a graph of speed versus time for accelerating from rest to 60mph in 6s, you would get a straight line pass through the origin to 60 mph. The area under this graph is the distance travelled by the object in 6 s.

It is found by calculating the area of a triangle:

$\frac{1}{2} \times 60 \text{ mph} \times 6 \text{ s}$

Before computing the whole thing do a bit of simplification,

$\frac{1}{2} \times 60 \text{ mph} \times 6 \text{ s} = 30 \text{ mph} \times 6 \text{ s}$

Actually, 30 mph is the average speed for the journey of accelerating from rest to 60mph in 6s.