Convert 24(decimal) to modified IEEE 754 floating point format?
Since $e=111$ is saved for indicating NaN and  $\infty$, then the maximal number that can be represented with this 8bit-floating point structure is
$$ (-1)^0 \cdot (1.1111)_b \cdot 2^{6-3} = 15.5$$

Therefore, you cannot represent 24(Dec) using the above system. 

However, the correct translation would be infinity rather than a NaN, that is, 24 should be represented as $0\ 111\ 0000 \equiv \infty$.