Number of hydrogen atoms per molecule of a hydrocarbon A having 85.8% carbon is (Given: Molar mass of A = 84 g mol$$^{-1}$$)

The molar mass of hydrocarbon (A) is (84\ \text{g mol}^{-1}), and it contains (85.8%) carbon.

The mass of carbon present in one mole of the compound is

$$84 \times 0.858 = 72.072 \approx 72\ \text{g mol}^{-1}.$$

Hence, the number of carbon atoms is

$$\frac{72}{12} = 6.$$

The remaining mass corresponds to hydrogen:

$$84 - 72 = 12\ \text{g mol}^{-1}.$$

Therefore, the number of hydrogen atoms is

$$\frac{12}{1} = 12.$$

Thus, the molecular formula of the hydrocarbon is

$$C_6H_{12}.$$

Hence, the number of hydrogen atoms present in one molecule of the hydrocarbon is

$$\boxed{12}.$$