ANSWER : x and ÷

Explanation : . If we interchange the signs x and ÷, the equation becomes 16 + 4 x 2 – 21 ÷ 7 = 16 + 8 – 3 = 21. Hence, option (4) is the correct answer.

ANSWER : 20

Explanation : . 16 + 64 ÷ 4 – 4 x 3 = 16 + 16 – 12 = 20

ANSWER : + and ÷

Explanation : . If we interchange signs + and ÷, then the equation results into 5 x 15 + 7 – 20 ÷ 4 = 5 x 15 + 7 – 5 = 77. Hence, option (3) is the correct answer.

ANSWER : ÷ and x

Explanation : . If we interchange signs ÷ and x, then equation changes into 4 ÷ 2 + 6 x 2 – 12 = 2 + 12 – 12 = 2. Hence, option (1) is the correct answer.