John spent $20 x 1/5 = $<<20*1/5=4>>4 on snacks.
He had $20 - $4 = $<<20-4=16>>16 remaining.
He spent $16 x 3/4 = $<<16*3/4=12>>12 on his necessities.
So, John is left with $16 - $12 = $<<16-12=4>>4.
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