1. （3-2.4×八分之五）÷三分之八
=(3-12/5×5/8)×3/8
=(3-3/2)×3/8
=3/2×3/8
=9/16
2. 六分之一+十二分之七÷（四分之三-六分之一）
=1/6+7/12÷(3/4-1/6)
=1/6+7/12÷(9/12-2/12)
=1/6+7/12÷7/12
=1/6+1
=7/6
3. 69÷（0.8+0.6×2.5）
=69÷(0.8+1.5)
=69÷2.3
=30
4. [1-（五分之二-十五分之一）]×60
=[1-(6/15-1/15)]×60
=(1-5/15)×60
=(1-1/3)×60
=60-20
=40