Answer
4503 seconds $\approx$ 1.25 hours
Work Step by Step
There are $2^{56}$ keys, one key can be checked in one floating point operations
The gaming computer can do 16 $\times10^{12}$ floating point operations per second
So time for checking $2^{56}$ keys will be -
time = $\frac{2^{56}}{16 \times10^{12}}$ seconds
time = 4503 seconds
