An equation that is very useful for this type of problem is called the partial sum, where all of the types of (whatever) equals to a certain amount of measurement based on a question. So in this case, an example would be "Total Pressure = Gas A + Gas B + Gas C + Gas D. Now substitute everything in, 860 mm Hg = 220 mm Hg + 220 mm Hg + 110 mm Hg + Gas D, leaving Gas D left to solve. Simplify it into 860 mm Hg = 550 mm Hg + Gas D, minus the 550 on both sides. This will now be 310 mm Hg = Gas D. So, the answer would be 310 mm Hg.
