Lesson 1: Round off to the nearest thousandth. Do these: (1) 13.48, (2) 7.123, (3) 14.46, (4) 9.369, (5) 0.887, (6) 0.946, (7) 0.8122, (8) 7.687, (9) 0.9122, (10) 8.466

$13.48 \approx 13.480$
$4.16 \approx 4.160$

$13.480 + 4.160 = 17.640$
The missing number is $\boxed{\text{17.640}}$.

2)

$7.123 \approx 7.123$
$5.6867 \approx 5.687$ (since the fourth decimal digit is 7, round up the third digit)

$7.123 + 5.687 = 12.810$
The missing number is $\boxed{\text{12.810}}$.

3)

$14.46 \approx 14.460$
$10.67 \approx 10.670$

$14.460 - 10.670 = 3.790$
The missing number is $\boxed{\text{3.790}}$.

4)

$9.369 \approx 9.369$
$4.1121 \approx 4.112$ (since the fourth decimal digit is 1, keep the third digit as is)

$9.369 + 4.112 = 13.481$
The missing number is $\boxed{\text{13.481}}$.

5)

$0.887 \approx 0.887$
$0.2845 \approx 0.285$ (since the fourth decimal digit is 5, round up the third digit)

$0.887 + 0.285 = 1.172$
The missing number is $\boxed{\text{1.172}}$.

6)

$0.946 \approx 0.946$
$0.631 \approx 0.631$

$0.946 - 0.631 = 0.315$
The missing number is $\boxed{\text{0.315}}$.

7)

$0.8122 \approx 0.812$ (since the fourth decimal digit is 2, keep the third digit as is)
$2.471 \approx 2.471$

$0.812 + 2.471 = 3.283$
The missing number is $\boxed{\text{3.283}}$.

8)

$7.687 \approx 7.687$
$5.232 \approx 5.232$

$7.687 - 5.232 = 2.455$
The missing number is $\boxed{\text{2.455}}$.

9)

$0.9122 \approx 0.912$ (since the fourth decimal digit is 2, keep the third digit as is)
$2.471 \approx 2.471$

$0.912 + 2.471 = 3.383$
The missing number is $\boxed{\text{3.383}}$.

10)

$8.466 \approx 8.466$
$0.322 \approx 0.322$

$8.466 - 0.322 = 8.144$
The missing number is $\boxed{\text{8.144}}$.