Divide 4.2 by 10.

While dividing a number by 10, 100 or 1000, the digits of the number and the quotient are the same but the decimal point in the quotient shifts to the left by as many places as there are zeros over 1.

$4.2÷10$

$=\frac{4.2}{10}$

$=\frac{42}{10}\times \frac{1}{10}$

$=\frac{42}{100}$

$=0.42$

Therefore, $4.2÷10=\mathbf{0}\mathbf{.}\mathbf{42}$