Question 899777
Assuming 30 days for a month, which you might later re-choose for an adjusted solution.  Also, assuming purchase of fares separately for going and for returning.


Regular bus fares, {{{2*2*30=120}}} dollars.


Coupon book fares, {{{2*2*d+24=120}}} dollars, using d days of non-discount bus usage days for matching the regular bus fares total.


{{{4d+24=120}}}
{{{d+6=30}}}
{{{d=30-6}}}
{{{d=24}}}
24 days without the coupon discount and 12 days with the use of the coupon book (because assuming 1 dollar either direction with the coupon book, so 12*2).