Train station Probability help

[prime-factor]

Homework Statement

Eighty per cent of all scheduled trains through
Roma Street station arrive on time. If 10 trains
go through the station every day, find:
a the probability that at least 8 trains are
on time
b the probability that at least 8 trains are
on time for 9 out of the next 10 days.

Homework Equations

P(X = x) = ⁿC_xp^xq^{n − x}

The Attempt at a Solution

P(X>=8)

n=10

x=8+9+10

p=0.8

q=0.2

P(X=8)+p(X=9)+p(X=10) = 0.6778

¹⁰C₈0.8⁸0.2² +¹⁰C₉0.8⁹0.2¹+¹⁰C₁₀0.8¹⁰0.2⁰

b I guess I would use:

P(X=8)+p(X=9)+p(X=10) = 0.6778

But I am not sure how. Please help!

 

[prime-factor]

worked it out:

set:

n=10

x=9

p= 0.6778

q=0.3222

And I get: 0.09728