Tính nhẩm:a) 15 cm2 + 20 cm2 = ..................60 cm2 - 42 cm2 = ..................20 cm2 + 10 cm2 + 15 cm2 = ..................b) 12 cm2 x 2 = ..................40 cm2 : 4 = ..................50 cm2 - 40 cm2 + 10 cm2 = ..................