Question 812706
Let {{{ x }}} = the 10 day supply of food for 15 horses
{{{ x/15 }}} = the amount of food per horse for 10 days
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{{{ 15 + 5 = 20 }}} horses now
Now the same amount of food means
{{{ x/20 }}} is the amount of food per horse
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Let {{{ d }}} = number of days the same 
amount of food lasts
Now I can set up the ratio:
{{{ ( x/15 ) / ( x/20 ) = 10/d }}}
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{{{ 20/15 = 10/d }}}
{{{ 20d = 150 }}}
{{{ d = 7.5 }}}
The same amount of food lasts 7.5 days