Tolong yaa...pakai cara

18.7 thn +20 thn -24 thn
=27 thn -24 thn
=3 thn (a)
19.(400 m +50dm) ×3
=(400 m +5 m) ×3
=405 m ×3
=1215 m (b)
20.2 rim 56 lembar -331 lembar
=1056 lembar -331 lembar
=725 lembar (b)