16.4 ÷ 10 = 1.64
The easiest way to divide decimals by 10 is to move the decimal point by one place to the left. It's the same if you divide by 100, but in that case you move it by two places. It's also similar with 1000, 10000 etc...
It also applies to multiplying, but in such situation you move the decimal point to the right.