Peter bought a computer which cost $249.in addition he had to pay a 10% sales tax on it. How much did peter pay altogether for the computer

To calculate the sales tax, we can multiply the cost of the computer by the tax rate: sales tax = $249 * 10% = $24.90

So, Peter paid $249 + $24.90 = $<<249+24.90=273.90>>273.90 altogether for the computer. Answer: \boxed{273.90}.