$$red:\ r,\ blue:\ b$$ $$Lance=3\ medium+3\ large$$ $$medium=5r+3b$$ $$large=10r+5b$$ $$Lance=3(5r+3b)+3(10r+5b)$$ $$=15r+9b+30r+15b$$ $$=45r+24b$$ $$\Rightarrow 24\ blue$$