The ratios are: oats:sugar:butter:syrup=9:6:3:4, by dividing the numbers by 20.
So, oats:sugar=9:6=3:2; oats:butter: 9:3=3:1.
12 flapjacks weigh 180+120+60+80=440g=0.44kg, one flapjack weighs 440/12=110/3=36⅔g.
18 flapjacks weigh 440(18/12)=440(3/2)=660g=0.66kg and use 1½ times each ingredient used to make 12 flapjacks: oats: 270g; sugar: 180g; butter: 90g; syrup: 120g.
60g of butter was used to make 12 flapjacks, so twice as much butter will make twice as many flapjacks (24), and the ingredients are: oats: 360g; sugar: 240g; butter: 120g; syrup: 160g. 24 flapjacks weigh 880g=0.88kg.