noun
1
BAD : I prefer to pay by cash.
GOOD : I prefer to pay in cash.
GOOD : I prefer to pay cash.
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pay by cheque , pay by credit card BUT pay in cash or just pay cash : 'If you pay (in) cash, you might get a discount.'
2
BAD : The purse contained about $200 cash.
GOOD : The purse contained about $200 in cash.
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amount of money + in cash : '£550 in cash', '$190 in cash'