Question 940319
Let {{{ s }}} = sales price
Let {{{ p }}} = original price
Let {{{ n }}} = number of jeans he buys
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{{{ s = n*( p - .3p ) }}}
{{{ s(n,p) = n*( .7p ) }}}
{{{ s(n,p) = .7n*p }}} 
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{{{ n = 4 }}}
{{{ p = 50 }}}
{{{ s(4,50) = .7*4*50 }}}
{{{ s(4,50) = 140 }}}
It cost him $140