Had a case study question, unfortunately I didn't finish it in time but it's still annoying me as I can't work it out. Any ideas?
There are $600,000$ users who log into an application an average of $11$ times per month. The average iPhone user logs in $7$ times per month; the average Android user logs in $13$ times per month
Question: What is the proportion of users who access the app via an iPhone?
So you have $u_A, u_I$ denote the proportion of Android and IPhone users, then $$ 11 = 7 u_I + 13 u_A $$ and $u_I + u_A = 1$. Solve and compute $u_I \times 600,000$.