Solution: In the X year, Xiaoming's family needs to pay 5,200 yuan.

5000+( 120000-30000)-5000(x-2)0.4% = 5200

5000+90000-5000 x+ 100000.4% = 5200

5000+( 100000-5000 x)0.4% = 5200

400-20x=200

x= 10

Method 2:

Judging from the meaning of the question, [120000-30000-5000 * (a-2)] * 0.04%+5000 = 5200.

The solution is a= 10.

∴a value should be 10.

Method 3:

Let's assume that the remaining debt of the previous year is X yuan.

5200-5000=0.4%X

200=0.4%X

X=5000

(120000-30000-5000)/5000 =17 (year)

The third method is different. I think you should use the first and second methods! Be safe!

But it's up to you!

I hope these three methods are useful to you!