Solution 10685: Troubleshooting ARITH ERROR on the TI-10 Elementary Calculator.
Why am I getting an "ARITH ERROR" when I divide a whole number by a decimal number on the TI-10?
The division MODE setting on the TI-10 can be set to "QR" (quotient remainder) or to "." (decimal).
If the TI-10 is in quotient remainder mode, then 5 ÷.5 will return the "ARITH ERROR" message. Setting the unit to decimal mode will return the correct solution (which is 10).
Please see the TI-10 guidebook for additional information.