Solution: If you buy X units, the prices of the two companies are exactly the same.

5800× 10+5800×70%×(x- 10)= 5800×85%×x

58000+4060x-40600=4930x

870x= 17400

x=20

A: When buying 20 sets, the prices of the two companies were exactly the same.

The format is completely correct, you must believe me, I am a sophomore!