Potential revenues of Sprinker

Potential Revenues of Sprinker

Pot. Revenues Sprinker ≈ 449k × $25 ≈ $11.22m

Hello community! 

I have a question regarding this part: the rest of the calculations are clear, but i would like to understand this: here we multiplied the (n. of private school children x price*book). However, I was wondering why we estimate that that number of private school kids (449k) is ALSO a first-grade kid. I fear I don't understand the correlation between the two data. 

Is it perhaps a hint the interviewer would give me upon request? As in, "to simplify, consider the n. of private school kids as the primary school kids".

Thank you!

Francesca