17.5+(x- 1)×2.5 = x÷2×4+x÷2×3，
? 17.5+2.5x-2.5=2x+32x，
15+2.5x=3.5x，
15+2.5x-2.5x=3.5x-2.5x，
x = 15；
17.5+( 15- 1)×2.5,
= 17.5+ 14×2.5,
= 17.5+35,
=52.5 (ten thousand yuan);
Answer: Mr. Li wants to pay 525 thousand to buy this house.