Physics 11 Astrophysics question

[love_maths]

Hello guys,

I'm stuck with this question please help

You are driving along a road at 100 km h-1 when you notice that
an aeroplane travelling in the opposite direction appears to be
stationary over a hill that is 2 km from you. In fact you know that
the aeroplane is actually travelling at 800 km h-1 in the opposite
direction. How far away from you is it?
Answer: 18km.

Thank you.

[love_maths]

While Hipparchus said it would take two +2 magnitude stars
to equal the brightness of one +1 star, the modern scale was
redefined so that it now takes 2.5 magnitude +2 stars to make
up one +1 star.

b) H ow many +6 stars are needed on the modern scale, and
why the difference?
2.5^5 = 97.6 but answer is 100. Why is that?