(1)$29.6 - 3.5 - 6.5=□◯(□◯□)$

29.6 - 3.5 - 6.5=29.6-(3.5+6.5)
=29.6-10
=19.6

(2)$8×2.5×12.5×4=(□◯□)◯(□◯□)$

$8×2.5×12.5×4=(8×12.5)×(2.5×4)$
$=100×10$
$=1000$

(3)$720÷45=□◯(□◯□)$

720÷45=720÷(9×5)
=720÷9÷5
=80÷5
=16

(4)$\frac{4}{7}×\frac{1}{3}+(\quad)×\frac{2}{3}=\frac{4}{7}◯(□◯□)$

$\frac{4}{7}×\frac{1}{3}+(\frac{4}{7})×\frac{2}{3}=\frac{4}{7}×(\frac{1}{3}+\frac{2}{3})$
$=\frac{4}{7}×1$
$=\frac{4}{7}$

2. 计算下面各题，能简算的要简算。
$3÷\frac{3}{4}-\frac{3}{4}÷3$
$4.6×5.2+0.48×46$
$(\frac{2}{3}+\frac{1}{6})÷\frac{1}{24}$
$3.6×99+3.6$
$0.25×32×1.25$
$546+283-146+317$

$3÷\frac{3}{4}-\frac{3}{4}÷3$
$=3×\frac{4}{3}-\frac{3}{4}×\frac{1}{3}$
$=4-\frac{1}{4}$
$=3\frac{3}{4}$
$4.6×5.2+0.48×46$
$=4.6×5.2+4.6×4.8$
$=4.6×(5.2+4.8)$
$=4.6×10$
$=46$
$(\frac{2}{3}+\frac{1}{6})÷\frac{1}{24}$
$=(\frac{2}{3}+\frac{1}{6})×24$
$=\frac{2}{3}×24+\frac{1}{6}×24$
$=16+4$
$=20$
$3.6×99+3.6$
$=3.6×99+3.6×1$
$=3.6×(99+1)$
$=3.6×100$
$=360$
$0.25×32×1.25$
$=0.25×(4×8)×1.25$
$=(0.25×4)×(8×1.25)$
$=1×10$
$=10$
$546+283-146+317$
$=(546-146)+(283+317)$
$=400+600$
$=1000$