(1) 90000 ÷ (6×12)+90000× 0.5% =1700 (yuan).

A: The repayment amount of Teacher Zhang in the first month after borrowing is 1700 yuan.

(2) The monthly average repayment amount of the loan principal is: 90,000 ÷ (12× 6) =1250 (yuan).

p = 1250+[90000-(n- 1)× 1250]×0.5% = 1706.25-6.25n；

(3) Substitute n=29.

p = 1250+[90000-(n- 1)× 1250]×0.5%

= 1250+[90000-(29- 1)× 1250]×0.5%

= 1250+[90000-35000]×0.5%

= 1525 yuan.

That is, the repayment amount of Mr. Zhang in July 20 10 is 1525 yuan.