All together, the two girls ate 8/15 of a pizza. you mulitply your fractions out of equal denominaters and get that.
All outcomes of three children (using G for girl and B for boy)
GGG, GGB,GBG,GBB,BGG,BGB,BBG,BBB
1) exactly two is 3 out of 8 total outcomes, or 3/8
2) all sons is one outcome, or 1/8