Problem 1 Estimate the weight of a 747 Problem 1 Estimate the weight of a 747 Ok 747 is a vehicle designed to carry people We dont know much about planes but lets think by proxy What other large vehicles ID: 756864
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Slide1
Google/Microsoft interviewSlide2
Problem 1
Estimate the weight of a 747. Slide3
Problem 1
Estimate the weight of a 747.
Ok, 747 is a vehicle designed to carry people.
We don’t know much about planes, but
let’s think by proxy. What other large vehicles
carry passengers?
Busses. Assume 747 = 6 buses. 4 in a row + 2 on top. Each bus ~ 60 people. So 747 ~ 360 people. Sounds about right. A bus weighs 5-10 tons. So, a 747 weighs between 6*5 and 6*10 = 30-60 tons.
(43 tons, actually). Slide4
Problem 2
In a country where couples want boys, they use the following strategy: couples have children until they have a boy, then stop.
What is the ratio of boys to girls in this country? Slide5
Problem 2
In a country where couples want boys, they use the following strategy: couples have children until they have a boy, then stop. What is the ratio of boys to girls in this country?
1
St
step: get your feet wet. Play with the problem. Start with N=2 couples. You can
have 2 boys. Or you can have 4 girls and then 2 boys. So, looks like all possible ratios are possible. Slide6
Problem 2
In a country where couples want boys, they use the following strategy: couples have children until they have a boy, then stop. What is the ratio of boys to girls in this country?
1
St
step: get your feet wet. Play with the problem. Start with N=2 couples. You can
have 2 boys. Or you can have 4 girls and then 2 boys. So, looks like all possible ratios are possible.
Of course, each of these outcomes has its own different probability. So, we could consider all outcomes and work out the probabilities. But that would be kind of difficult…Slide7
Problem 2
In a country where couples want boys, they use the following strategy: couples have children until they have a boy, then stop. What is the ratio of boys to girls in this country?
Alternative. Let’s go to the large N limit. Appropriate for “country”. Will simplify things. Exactly ½ couples will have boys, ½ will have girls.
First, what is easier to find, N
B
or N
G
? Of course, N
B
= N. So, need to focus on N
G
Draw a BIG diagram. Slide8
Problem 2
couples
Offspring generation #
N/2
N/2
BBBBBBBBBBBBB GGGGGGGGGGGGG
1
2
3
BBBBBB GGGGGG
N
/4
N
/8
BBB GGG
Count girls: N
G
= N/2 + N/4 + N/8 + … = N = N
B