Page 45: Three. 1, × 2, × 3, × 4, × 5, √ 6 and √.
Four. 1 21-15+9 =15 (only) 2,12 ÷ 6 = 2 (6+12) ÷ 2 =
Page 46: Two. 1，80-23- 17 = 40(m)80-(23+ 17)= 40(m)
2.4×6+27=5 1 (branch)