## Invitation to Computer Science 8th Edition

$78$ gifts --- We are interested in the number of gifts given on Day 12 for which we have 12 verses where each $n^{t h}$ verse adds $n$ gifts to the total amount. We can use the formula that is already known: $1+2+3+\ldots+n=\frac{n \cdot(n+1)}{2}$ Replacing 12 for $n$ in the formula yields $\frac{12 \cdot(12+1)}{2}=78$ So the number of gifts given on Day $12$ is $78 .$