Chemical equilibrium

In most of the solutions, they've simply taken x=partial pressure of B (refer image 2). But isn't partial pressure of B = [x/(1-x)]×total pressure (basically the partial pressure of gas equal to mole fraction × total pressure). We could take x as concentration of B at equilibrium (assuming volume to be 1L) but how can we take it as the partial pressure?

The solution in the 3rd image seems more logical, but they've made the assumption that pressure is 1 atm.

What's your take?