How does the swap trick combine with the 10%/1% anchors for something like 40% of 65?

Either build 40% from 10%s, or swap — pick whichever is cleaner.

• Build: 10% of 65 = 6.5, ×4 = 26.
• Swap: 40% of 65 = 65% of 40 = (50% + 10% + 5%) of 40 = 20 + 4 + 2 = 26.

Check: both give 26. ✓ Good mental math is often having two routes and taking the shorter one; the anchors, the swap, and the fraction table are the three tools you mix.