Question 901301
Let {{{ n }}} = number of riders that  
were on the bus before the first stop
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1st stop:
{{{ n - 10 }}} after 10 got off
{{{ n - 10 + 15 = n + 5 }}}
after 15 got on
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2nd stop:
{{{ n + 5 - 8 = n - 3 }}}
after 8 got off
{{{ n - 2 + 20 = n + 18 }}}
after 20 got on
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3rd stop:
{{{ n + 18 - 18 = n }}}
after 18 got off
{{{ n + 3 }}} 
after 3 got on
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You are told that
{{{ n + 3 = 13 }}}
{{{ n = 10 }}}
There were 10 riders were on 
the bus before the first stop