Answer
0.77 seconds
Work Step by Step
There are $2^{56}$ keys, one key can be checked in one floating point operations
The gaming computer can do 93 ×10$^{15}$ floating point operations per second
So time for checking 256 keys will be -
time = $\frac{2^{56}}{93×10^{15}}$ seconds
time = 0.77 seconds
