If P = +, Q = −, R = ×, S = ÷, then insert the proper notations between the successive numbers in the equation 60_15_3_20_4 = 20:

Testing option (a) SPRQ: 60 S 15 P 3 R 20 Q 4 = 60 ÷ 15 + 3 × 20 − 4 = 4 + 60 − 4 = 60. That gives 60, not 20. Testing (b) QRPS: 60 − 15 × 3 + 20 ÷ 4 = 60 − 45 + 5 = 20. ✓ This gives 20. Therefore the correct notation sequence is QRPS, option (b). Rechecking official key answer: option (b) QRPS yields 20 exactly, matching the required result.