Place $1$ through $9$ in the nine cells below, using each once. Both the vertical and horizontal lines sum to $23$. What number occupies the central cell?
There are $28$ birds on two branches. Four fly from the first to the second, then seven fly from the second to the first, leaving equal numbers on the branches. How many were originally on the first branch?
Three books—Literature, Mathematics and Science—have blue, red and yellow covers. The red book lies between the Literature and Science books. The Science book and the blue book were bought on the same day. What is each book’s cover color?
Pi thinks of a four-digit natural number. One digit is $1$; the hundreds digit is three times the thousands digit; the units digit is four times the tens digit; and the thousands digit is $2$. What is the digit sum?
Eight boxes each contain at least one pebble. All counts are different except for exactly two boxes with the same count. What is the smallest total number of pebbles?
An has clocks $A , B$ and $C$. A gains $30$ seconds per hour compared with $B$, while $B$ loses $30$ seconds per hour compared with $C$. All are reset to $8$ a.m. What does A show when $C$ shows noon?
Eight cards are numbered $1$ through $8$. An, Binh and Chau each draw two cards.
• Binh: one card’s number is twice the other’s.
• Chau: the two numbers sum to $9$.
• An: the two numbers sum to $7$, but their difference is not $3$.
Which two cards remain?