Time taken for Brute Force Attack on DES An attacker is equipped with a computer that performs 10 trillion (1013) DES decryptions per second, what is the average time required, in hours, for a brute force attacker to break DES? Key size is 56 bits.

I tried 256/1013 divided by 3600 but the answer is incorrect.
You got $2^{56}$ different keys and the attacker can test $10^{13}$ keys per second.

In the worst case, the last tested key is the solution. For that you need $2^{56} / 10^{13} \approx 7205$ sec. -> around $2$ hours.

In the best case, the first tested key is the solution. This takes about 1 second.

On Average you need to test half of the keys. That means: $0.5 * (2^{56} / 10^{13}) \approx 1$ hour.