(1) class b: 70×2= 140 yuan.

So 189- 140=49 yuan.

(2) Buy X for the first time and 70-x for the second time.

It is discussed in three situations.

( 1) x < 20，70 x > 50。

Then 3x+2(70-x)= 189.

The solution is x=49 (it does not meet the range of x value, so it is discarded).

② 20 < x < 30 and 40 < 70-x < 50.

The equation is 3x+2.5(70-x)= 189.

The solution is x = 28- consistent! ! !

③ 30 < x < 35 (because the second time is greater than the first time), then 35 < 70-x < 40.

The equation is 2.5x+2.5(70-x)= 189.

X has no solution

To sum it up. There is x=28,

Answer: I bought 28 kg for the first time and 42 kg for the second time!

Is that clear ~ ~:) Here you are ~ ~ Hehe.