Question 129417
Think about her money like a pie.
She starts with a whole pie. 
She gave 1/5 of the pie for snacks.
She gave 1/4 of the pie for transport. 
She gave 1/8 of the pie for club fees.
You're correct she gave away 23/40.
How much pie did she start with? 1.
If she gave away 23/40, the remainder (what's left) is the difference between the two, 
{{{R=1-23/40}}}
{{{R=40/40-23/40}}}
{{{R=(40-23)/40}}}
{{{R=17/40}}}