One of the following four-digit numbers is not divisible by 4: 3544, 3554, 3564, 3572, 3576. What is the product of the units digit and the tens digit of that number?
To check if a four-digit number is divisible by 4, I only need to look at its last two digits. If the last two digits form a multiple of 4, then the whole number is divisible by 4, and vice versa. So I can ignore the first two digits and just focus on the last two. I can quickly see that 3544, 3564, 3572, and 3576 are all divisible by 4, because 44, 64, 72, and 76 are multiples of 4. The only number that is not divisible by 4 is 3554, because 54 is not a multiple of 4. Therefore, the product of the units digit and the tens digit of that number is 5 * 4, which is 20. 