Question 934484
Let {{{ x }}} = the number of fish he started with
He gives {{{ ( 2/3 )*x }}} of his fish to the high school. 
That means he has {{{ ( 1/3 )*x }}} left
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he gives {{{ 2/5 }}} of the remaining fish to the elementary school.
{{{ ( 2/5 )*( 1/3 )*x }}} goes to the elementary school
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He has {{{ 30 }}} fish left
This is telling you that:
{{{ ( 1/3)*x - (2/5)*(1/3)*x = (3/5)*(1/3)*x }}}
is the number of fish he has left and
{{{ (3/5)*(1/3)*x = 30 }}}
{{{ (3/15)*x = 30 }}}
{{{ (1/5)*x = 30 }}}
{{{ x = 150 }}}
He started with 150 fish
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check:
{{{ (2/3)*150 = 100 }}} goes to the high school
He has {{{ 50 }}} left
{{{ (2/5)*50 = 20 }}} goes to the elementary school
He has {{{ 30 }}} left
OK