估算下面各題（中間一步，以后熟練了，就可以不寫出來。）
21×2≈ × = 32×2≈ × =
43×2≈ × = 54×2≈ × =
56×2≈ × = 57×2≈ × =
58×2≈ × = 59×2≈ × =
31×3≈ × = 42×3≈ × =
53×3≈ × = 64×3≈ × =
76×3≈ × = 87×3≈ × =
98×3≈ × = 99×3≈ × =
41×4≈ × = 52×4≈ × =
63×4≈ × = 74×4≈ × =
86×4≈ × = 97×4≈ × =
98×4≈ × = 99×4≈ × =
51×5≈ × = 62×5≈ × =
73×5≈ × = 84×5≈ × =
96×5≈ × = 97×5≈ × =
98×5≈ × = 99×5≈ × =
61×6≈ × = 62×6≈ × =
73×6≈ × = 74×6≈ × =
86×6≈ × = 97×6≈ × =
98×6≈ × = 99×6≈ × =
71×7≈ × = 82×7≈ × =
93×7≈ × = 94×7≈ × =
96×7≈ × = 97×7≈ × =
98×7≈ × = 99×7≈ × =
81×8≈ × = 82×8≈ × =
83×8≈ × = 84×8≈ × =
86×8≈ × = 87×8≈ × =