Question 1106747
Note per hour is per two half-hours.


{{{y=mx+b}}}, what you charge, y.


If r is rate per half-hour, then 2r is rate per 1 hour; and this means, m=2r.


The quantity to charge just for show up by itself is b.


Your description gave no values;
{{{y=(2r)x+b}}} for x number of HOURS, and rate per HOUR of 2r.