Multi-digit multiplication and division
Split the larger number into its place-value parts, work with each part, then add the partial answers.
A worked example
For 34 x 21, do 34 x 20 = 680 and 34 x 1 = 34, then add: 680 + 34 = 714.
The mistake people actually make
In long division, write a zero in the quotient when a place will not divide, or the answer comes out ten times too small.