CAT 2025 Slot2 Quant: The average number of copies of a book sold per day by a shopkeeper is 60 in the initial seven days

Step 1: Find Sales on the 8th Day
Total sales in first 7 days $= 7 \times 60 = 420$.
Total sales in first 8 days $= 8 \times 63 = 504$.
Sales on 8th day $(D_8) = 504 - 420 = \mathbf{84}$.

Step 2: Find Sales on the 9th Day
It is given that 9th day sales are 11 less than 8th day.
Sales on 9th day $(D_9) = 84 - 11 = \mathbf{73}$.

Step 3: Calculate the Sum for Days 2 to 9
Average from Day 2 to Day 9 (8 days total) is 66.
$\sum_{i=2}^{9} D_i = 8 \times 66 = \mathbf{528}$.

Step 4: Find Sales on the 1st Day
Total sales from Day 1 to Day 9 $= (\text{Total first 8 days}) + D_9$
$\sum_{i=1}^{9} D_i = 504 + 73 = 577$.
Sales on Day 1 $= (\sum_{i=1}^{9} D_i) - (\sum_{i=2}^{9} D_i)$
$D_1 = 577 - 528 = \mathbf{49}$.