Make equal: 1. 20 + 5 + ... = 35; 2. 15 + 15 + ... = 45; 3. 3 x 5 = 7 + ...; 4. double 25 = 67 - ...; 5. 7 x 4 = 14 x ...; 6. 6 x 3 = 85 - ...; 7. 36 / 3 = 20 - ...; 8. ... + 75 = 125 − 25

$$
20 + 5 = 25
$$

$$
35 - 25 = 10
$$
The missing number is $\boxed{\text{}10\text{}}$

$$
15 + 15 = 30
$$

$$
45 - 30 = 15
$$
The missing number is $\boxed{\text{}15\text{}}$

$$
3 \times 5 = 15
$$

$$
15 - 7 = 8
$$
The missing number is $\boxed{\text{}8\text{}}$

$$
2 \times 25 = 50
$$

$$
67 - 50 = 17
$$
The missing number is $\boxed{\text{}17\text{}}$

$$
7 \times 4 = 28
$$

$$
28 \div 14 = 2
$$
The missing number is $\boxed{\text{}2\text{}}$

$$
6 \times 3 = 18
$$

$$
85 - 18 = 67
$$
The missing number is $\boxed{\text{}67\text{}}$

$$
36 \div 3 = 12
$$

$$
20 - 12 = 8
$$
The missing number is $\boxed{\text{}8\text{}}$

$$
125 - 25 = 100
$$

$$
100 - 75 = 25
$$
The missing number is $\boxed{\text{}25\text{}}$