super simple!
2(150)+1(120)=420/3=140 pounds! the average of a set of integers will remain the same no matter how many integers (runners in this case) are involved as long as the ratios remain the same.
Sometimes I like to solve these problems with a little shortcut I made up (disclaimer: I'm sure this strategy is older than the GMAT, but I've never read it anywhere)
120-----150
there is a difference of 30 pounds and a 2:1 ratio between men:women. Put 2:1 into the 30 pound difference 20:10. 120+20=140 or 150-10=140. You think "there are more men then women, so the average will be much closer to the heavier side." then it's just applying the ratio to the difference to get specific.
sometimes this strategy is easier, sometimes it's harder. take it on a case by case basis