Here's a one that's a bit physics and a bit maths but maybe too easy.
1/R=1/R1 +1/R2 for parallel resistors.
How do you produce examples of this with whole number values only?
e.g. R1=14, R2=35 giving R=10.
R1=21, R2=28, R=24 gives exactly R=8.
R1=1400, R2=2600 produces 910 ohms.