The idea here is that the total pressure of the gaseous mixture will be equal to the sum of the partial pressures of its components as given by Dalton's Law of Partial Pressures.
So you can say that you have
#P_"total" = sum_i P_i#
In your case, the mixture is said to contain three gases, each with a known partial pressure. You can thus say that in order to get the total pressure of the mixture, you need to add the three partial pressures.
#P_"total" = P_ ("H"_ 2) + P_( "CO"_ 2) + P_ ("CH"_ 4)#
You will end up with
#color(darkgreen)(ul(color(black)(P_"total"))) = "200 torr" + "150 torr" + "320 torr" = color(darkgreen)(ul(color(black)("670 torr")))#