Mandatory Digit

A digit that appears in every partition of a run must be somewhere in that run. If only one cell can take it, it goes there.

Level 2 Single digit in a run Technique 07

Why it works

Some digits are unavoidable. Every way to make 17 in two cells uses 8 and 9. Every way to make 10 in four cells uses 1, because the other three digits cannot reach 10 without it.

Find the digits common to every partition, then look at where each one can go. A mandatory digit with a single possible cell is a placement.

How to spot it

1. List the partitions of the remaining sum for the number of empty cells.
2. Keep the digits that appear in all of them.
3. For each such digit, count the empty cells that can hold it. One cell means a placement.

Practice it

Open the app, choose a level that uses this technique, and ask the coach for the next step. Every hint links back to this guide.