It cost $420 to print flyers encouraging people to vote yes for the rights of indigenous people. The printing company prints the order for 1

10% off means they will pay 90% of the basic cost.  90% of 420 is

$${\frac{{\mathtt{90}}}{{\mathtt{100}}}}{\mathtt{\,\times\,}}{\mathtt{420}} = {\mathtt{378}}$$   so they pay $378

Alternatively, you can take 10% of $420 (that is, one-tenth of $420), namely $42, and subtract this from $420.

$420 - $42 = £378