Explanation:

Correct option (4): − and ×

Step 1: Evaluate the original expression (without changes)

Following BODMAS (or PEMDAS):

1. Division and multiplication from left to right:

   48 ÷ 12 = 4
   4 × 3 = 12

2. Add and subtract from left to right:

   12 + 5 = 17
   17 − 4 = 13

So the original expression equals 13, not 21.

Step 2: Try different interchanges to see what makes the result = 21

Try interchanging ÷ and −

Expression becomes:

48 − 12 × 3 + 5 ÷ 4

Step-by-step:

• 12 × 3 = 36

• 5 ÷ 4 = 1.25

• 48 − 36 = 12

• 12 + 1.25 = 13.25  Not Correct 

Try interchanging ÷ and +

Expression becomes:

48 + 12 × 3 ÷ 5 − 4

• 12 × 3 = 36

• 36 ÷ 5 = 7.2

• 48 + 7.2 = 55.2

• 55.2 − 4 = 51.2  Not correct

Try interchanging ÷ and ×

Expression becomes:

48 × 12 ÷ 3 + 5 − 4

• 48 × 12 = 576

• 576 ÷ 3 = 192

• 192 + 5 − 4 = 193  Not Correct

Try interchanging + and −

Expression becomes:

48 ÷ 12 × 3 − 5 + 4

• 48 ÷ 12 = 4

• 4 × 3 = 12

• 12 − 5 = 7

• 7 + 4 = 11  Not Correct

Try interchanging × and −

Expression becomes:

48 ÷ 12 − 3 + 5 × 4

• 48 ÷ 12 = 4

• 4 − 3 = 1

• 5 × 4 = 20

• 1 + 20 = 21 Correct 

So, 

Interchange × and −

So the signs to be interchanged are:
 × (multiplication) and − (subtraction)

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