Aliens who live 1,000,000 miles from Earth decide to come to our plane

\(Speed(ave.)= (D_1+D_2+...D_n)/(T_1+T_2+...+T_n)\)

\(D_1=500000 ml\)
\(Speed_1=100000 mph\)
--> \(Time_1= \frac{500000}{100000}=5 h\)

\(D_2=250000\) ml
\(Speed_2=200000\) mph
--> \(Time_2=\frac{5}{4}\)

\(D_3=125000\) ml
\(Speed_3= 400000\) mph
--> \(Time_3= \frac{5}{16}\)

As you see the Distance and Time, they are creating the Infinite terms of Geometric Progressions series (\(S_∞=\frac{b}{(1-q)}\) )
--> b is the first term, q is the common ratio (\(\frac{b_n}{b_{n-1}}\))

Distance:

b= 500000
\(q = \frac{b_2}{b_1}= \frac{250000}{500000}= \frac{1}{2}\)
--> \(S_∞= \frac{500000}{(1-1/2)}= 1000000\)

Time:

b=5
\(q= \frac{b_2}{b_1}= \frac{1.25}{5}= \frac{1}{4} \)
--> \(S_∞= \frac{5}{(1-1/4)}= \frac{20}{3}\)

\(Speed(ave.)= \frac{Distance}{Time}= \frac{1000000}{(20/3)}= 150000\)

The answer is D