n = number of people originally
175/n +10 = 175/(n-2)
(175+10n)/n = 175/(n-2)
Cross multiply: (n-2)(175+10n) = 175n
10n^2 -20n +175n - 350 = 175n
Subtract 175n: 10n^2 -20n-350=0
Divide: n^2-2n-35=0
Factor: (n-7)(n+5)=0
Solutions are -5 and 7, but the only plausible answer is 7 people at first