Multiply: $3.4\times 4.2$

We have,

$3.4\times 4.2$

$=\frac{34}{10}\times \frac{42}{10}$

$=\frac{34\times 42}{10\times 10}$

$=\frac{1428}{100}$

On dividing a number by 100, the decimal point is shifted to the left by two places.

Therefore,

$3.4\times 4.2=\frac{1428}{100}=\mathbf{14}\mathbf{.}\mathbf{28}$