"A piece of land has 5 mu, with an average yield of 450 Jin per mu", so the yield of a piece of land is 450 times 5, which is equal to 2250 Jin.

"A and B wheat fields have an average yield of 420kg/ mu", so the average yield of A and B wheat fields is 420 times 5 = 2 100kg/ mu.

It can be seen that the average yield of 5 mu in plot A is 2250-2 100= 150 kg more than that in plots A and B.

The average yield per mu in area B is 420-400 = 20kg less than that in areas A and B..

Then plot B has 150 divided by 20=7.5 mu.

I hope this helps.