Question 622961
Let {{{ t }}} = the time in minutes to arrive on schedule 
Note that 15 min is 1/4 hr
{{{ t + 15 }}} = the actual time train arrived
{{{ 100 = 60*( t + 1/4 ) }}}
{{{ 100 = 60t + 15 }}}
{{{ 60t = 100 - 15 }}}
{{{ 60t = 85 }}}
{{{ t = 17/12 }}} hrs
To arrive on time, let speed = {{{ s }}}
{{{ 100 = s*(17/12 ) }}}
{{{ s = (12/17)*100 }}}
{{{ s = 70.6 }}} mi/hr
I don't get the exact answers, but I 
don't see any mistake- do you?