(a) The first purchase of Class A is "no more than 30 kg" and the second purchase is "no more than 30 kg but no more than 50 kg".

(2) That is, Class A purchases "more than 30 kilograms but not more than 50" for the first time and "more than 30 kilograms but not more than 50" for the second time.

(three) that is, the first purchase of Class A is "no more than 30 kilograms" and the second purchase is "more than 50 kilograms"

So according to three situations,

(1) Solution: If Class A buys X kilograms for the first time and Y kilograms for the second time, then

3x+2.5y= 189( 1)

x+y=70(2)

solve an equation

(2)*3-( 1)

0.5y=2 1

y=42

Substitute y=42 into (2).

x=28

therefore

x=28

y=42

A: Class A bought 28 kg for the first time and 42 kg for the second time.

(2) Solution: If Class A buys X kilograms for the first time and Y kilograms for the second time, then

2.5x+2.5y= 189( 1)

x+y=70(2)

solve an equation

X+y=75.6 from (1)

It doesn't conform to Equation 2, so it doesn't hold.

(3) Solution: If Class A buys X kilograms for the first time and Y kilograms for the second time, then

3x+2y= 189( 1)

x+y=70(2)

solve an equation

(2)*3-( 1)

y=2 1

Substitute y=2 1 into (2).

X=49, so

x=49

y=2 1

Answer: Class A bought 49 kg for the first time and 2 1 kg for the second time.