IB Mathematics SL Prep

Answer: D - 15 x 15 is 225 square feet.

Key Takeaway: To square any number that ends in 5, use this trick.

Take the first digit of the number, in this case 1. Multiply that by itself and the next consecutive digit, which is 2.

So far, we have 1 x (1+1) = 1 x 2 = 2. Then, simply put 25 on the end, which results in 225. This trick works for squaring any number that ends in 5.