Question 1114642
Let {{{ x }}} = amount he should give sister
{{{ x - 10 }}} = amount that his sister ended up 
with after she paid $10 to friend
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given:
{{{ 93 - x = x - 10 }}}
{{{ 2x = 103 }}}
{{{ x = 51.5 }}}
Phil should give his sister $51.50
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check:
{{{ 93 - 51.5 = 51.5 - 10 }}}
{{{ 41.5= 41.5 }}}
OK