$A = 3 \times 3$ square has equal row, column and diagonal sums. Given $64$ top-left, $6$ top-middle, $14$ bottom-right, and central A as pictured, find $A$.
Ten coins start on one face. Four rounds flip $3 , 5 , 7 , 9$ coins, ending all on the opposite face. What maximum number are flipped exactly three times?
A machine divides an even number by $2$ and subtracts $1$ from an odd number, stopping at $0$. Find the smallest number requiring exactly seven steps. Example $6$→$3$→$2$→$1$→$0$ takes four steps.
Matchsticks form $2021$ as shown. Move exactly three sticks, without adding or removing, to make the largest four-digit number using the pictured digit patterns. Find it.
Rectangle $ABCD$ is split into $AEFD$ and $BCFE$ as pictured. $BCFE$ has twice the perimeter and four times the area of $AEFD$. $AD = 10 \,\mathrm{cm}$. Find $AB$.
Twenty students’ mean age is half the teacher’s age. Including the teacher, the class mean is $20$ years below the teacher’s age. Find the teacher’s age.