Question 263404
If I understand the question correctly, we have two equations
(i) {{{15(m-60)}}}
(ii) {{{10(m-60)+40}}}
setting them equal, we get
{{{15(m-60) = 10(m-60)+40}}}
distribute to get
{{{15m - 900 = 10m - 600 + 40}}}
combine like terms to get
{{{15m - 900 = 10m - 560}}}
subtract 10m and add 900 to get
{{{5m = 340}}}
divide to get
{{{m = 68}}}
So, at 68 mph, 
city A fine will be $120
city B fine will be $120