Ask HN: Apart from programming, how have you used algebra in the real world?

My son's struggling a bit with algebra and asked the age-old question - "when will I use this?" He wasn't being sulky, just genuinely confused.

My instinct was to say programming but after that I was stuck. When he was learning percentages I used shopping offers and discounts as a concrete example of that being useful. What's a similar example for algebra?