Question 67
Pyramids with square bases were built by stacking magnetic balls. A pyramid with a $1 \times 1$ base contains one ball, a pyramid with a $2 \times 2$ base contains five balls, a pyramid with a $3 \times 3$ base contains 14 balls, a pyramid with a $4 \times 4$ base contains 30 balls. How many balls are there in a pyramid with a $10 \times 10$ base?