Calculator

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He punched the digits into his calculator and turned it upside down. He roared like a lunatic at his own hilarity.
The boy next to him turned around and frowned. The frown became a smile once he saw the calculator's screen.
"What's going on here?" the teacher asked, striding towards their table.
The attempt to remove the calculator from sight failed and the teacher snatched it up.
"Ah yes. Very funny," he said.
The teacher punched away on the keys and then turned the calculator upside down to show the boys.
"STOP PISSING ABOUT," it said.
The boys tried to figure out which digits had been used.