Assumptions & Clarifications

Its a typical weekday. Not a holiday. Sometime in late Spring or early Fall so that its not tied to seasonal variations. For e.g, not during summer vacations, or spring break, winter break etc.

 

Equation

Passengers arriving = number of flights landing in SFO x avg number of people in a flight.

 

Number of flights landing = number of gates x number of flights landing per gate.

 

Number of gates = Number of terminals x number of gates per terminal

 

Terminals = I think 3 terminals.

Number of gates per terminal = 40 gates.

Total number of gates = 120.

 

Number of flights landing per gate dependent time of day, turn around time for flight to deboard and board passengers. Turn around time = 1hr.

7am  – 11am: High. 100% gates open. No time wasted between flights within a gate. 1 flight per hr = 4 flights per gate.  120*4 = 480.

11am –  4pm:  Medium: 75% gates open. 1 flight per hr = 5 flights per gate. Flights = 120 * 5 * 0.75 = 450.

4pm – 9pm: High. 100% gates open. 4 flights per gate. Flights = 120 *5 = 600.

9pm – 7am: Low. 20% gates open. 1 light every 2 hrs per gate. Flights = 10 * 0.2 * 0.5 * 120 = 120.

Total flights landed = 1750 flights.

 

Number of people per flight. Assume occupancy = 75%

Type of flights: Large: 400 people. 20%

Medium: 200 people. 50% flights. Here assume 20% of flights are stopping with a layover in SFO. So only 100 people will disembark.

100*0.1 + 200*0.4

Small: 80 people. 30% flights.

 

Avg number of people = 0.75 * (0.2*400 + 100*0.1 + 200*0.4 + 80*0.3) = 0.75*(80 + 90 + 24) = 0.75*200 = 150 people.

 

Total people landing = 150 * 1750 = 262K passengers landing.

Answering this Google estimation interview question is all about choosing the right numbers. Clarify these upfront with the interviewer.

Clarifying Questions:

By arriving people count we mean people arriving by flight specifically into SF airpoirt.

Equation:

# of people arriving = (# of planes arriving in SF x average capacity of each plane)

Assuming average capacity of each plane = 150 passengers

# of planes arriving in SF = # of planes arriving in SF during peak hours + # of planes arriving in SF during non peak hours

# of planes arriving in SF during peak hours = number of runways in SF x (average # planes during peak)

# of planes arriving in SF during non peak hours = number of runways in SF x (average # planes during non peak)

(average # planes during peak) –> assuming there is atleast a 5 min gap per plane landing —> 60 mins / 5 –> 12 planes per hour x peak duration

(average # planes during non peak) –> assuming there is atleast a 15 min gap per plane landing —> 60 mins / 15 –> 4 planes per hour x non peak duration

Peak duration 6 am – 10 pm –> 16 hours

Non Peak duration 6 am – 10 pm –> 8 hours

# of runways in SF airport = 2 (just remember seeing it from the sky 🙂 )

# of planes arriving in SF = (2 x 12 x 16) + (2 x 4 x 8) –> ~450 planes in 24 hours

assuming every plane has a capacity of 200 passengers –> 450 planes x 200 passenger/plane –> 90000 passengers arrive in SF everyday