Here's how to solve this problem using Dalton's Law of Partial Pressures:

Dalton's Law of Partial Pressures

Dalton's Law states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the individual gases.

Formula

P_total = P_A + P_B + P_C + ...

where:

* P_total is the total pressure of the gas mixture

* P_A, P_B, P_C, ... are the partial pressures of each individual gas

Applying the Law to the Problem

1. We know:

* P_total = 1.20 atm

* P_A = 0.75 atm

2. We want to find:

* P_B

3. Using Dalton's Law:

1.20 atm = 0.75 atm + P_B

4. Solving for P_B:

P_B = 1.20 atm - 0.75 atm

P_B = 0.45 atm

Therefore, the partial pressure of gas B is 0.45 atm.