Question 957531
Let {{{ n }}} = the number of machines
he purchased
{{{ 280/n }}} = the price for 1 machine
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{{{ 280/( n - 4 ) = 288/n }}} 
Multiply both sides by {{{ n*( n-4 ) }}}
{{{ 280n = 288*( n - 4 ) }}}
{{{ 280n = 288n - 1152 }}}
{{{ 8n = 1152 }}}
{{{ n = 144 }}}
He bought 144 machines
check:
{{{ 280/( 144 - 4 ) = 288/144 }}} 
{{{ 280/140 = 288/144 }}}
{{{ 2 = 2 }}}
OK