Question 561189
Let {{{ x }}} = number attending 2nd school
{{{ .4*900 = 360 }}} in 1st school wearing coat
{{{ .75x }}} in 2nd school wearing coat
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{{{ .525*( 900 + x ) }}} are wearing a coat
{{{ 360 + .75x = .525*( 900 + x ) }}}
{{{ 360 + .75x = 472.5 + .525x }}}
{{{ .75x - .525x = 472.5 - 360 }}}
{{{ .225x = 112.5 }}}
{{{ x = 500 }}}
500 students attend the 2nd school
check answer:
{{{ 360 + .75x = .525*( 900 + x ) }}}
{{{ 360 + .75*500 = .525*( 900 + 500 ) }}} 
{{{ 360 + 375 = .525*1400 }}}
{{{ 735 = 735 }}}
OK