Question 763201
Suppose Jason bough 20 pencils
{{{ 20*5 = 100 }}} 
He has $20 to spend on the rest
{{{ 9 + 6 + 4 = 19 }}}
That works, since $119 is under $120
and it is a prime number
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21 pencils means he spends {{{ 21*5 = 105 }}}
and has $15 left
{{{ 9 + 6 + 4 = 19 }}}
He ends up spending $124, 
which is more than $120
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The most pencils he can buy is 20