36/60*X=30/60*(X+40)

The solution is X=200.

② Set the junior high school plan as X and the senior high school plan as Y..

X+Y=3000

20%*X+30%*Y=3780-3000

X= 1200 Y= 1800。

③ let wheat x corn y.

X+Y= 18

12% * X+ 10% Y = 20- 18

X= 10 Y=8。

④ Let Dennis spend Q and Carrefour spend W..

Q=300+(X-300)*80%

W=200+(X-200)*85%

The two companies use the same fees.

Q=W

300+(X-300)* 80% = 200+(X-200)* 85%

X=600

Please ask questions if you have any questions, and please adopt them if you have no questions! ! Thank you!