The answer is 75.
The trick relies on multiplying the two numbers together and then adding a sequence of descending numbers (7, 6, 5, 4, 3…) to the product.
Here is the pattern applied to each row:
5 + 7 = 42: (5 × 7) + 7 = 35 + 7 = 42
6 + 8 = 54: (6 × 8) + 6 = 48 + 6 = 54
4 + 9 = 41: (4 × 9) + 5 = 36 + 5 = 41
7 + 8 = 60: (7 × 8) + 4 = 56 + 4 = 60
Following this exact descending sequence, the number to add for the final equation is 3: