Find the formula with the following composition: H-2.1%; N-29.8%; O-68.1%

Let us denote: a is the number of hydrogen atoms, b is the number of nitrogen atoms, c is the number of oxygen atoms, M is the molecular weight of the compound.
The relative atomic mass of hydrogen is 1, nitrogen is 14, oxygen is 16.
Let’s compose the following expressions:
0.021 = 1 * a / M;

0.298 = 14 * b / M;

0.681 = 16 * c / M;

Let us express the molecular weight from three expressions:
M = 1 * a / 0.021 = 47.6a;

M = 14 * b / 0.298 = 47b;

M = 16 * c / 0.681 = 23.5c;

Approximate values of unknown numbers: a = 1, b = 1, c = 2. The formula of the substance is HNO2.
Answer: the formula of the substance is HNO2.