20 times 5 is 100. We can multiply 2*5 and get 10, then add another zero to make it easier.
<span>This property is called associativity. It shows that it doesn't matter whether you perform the sum on the first two numbers and then the third, or on the second two and then the first, because you will get the same answer either way. That is because addition is associative.</span>
12! / 4!
= 1×2×3×4×5×6×7×8×9×10×11×12 / 1×2×3×4
= 5×6×7×8×9×10×11×12 = 19958400
