Grape accounts for 1/5 of the four fruits.

Watermelon planting area accounts for 50% of the total planting area of grapes, loquat and citrus.

Watermelon accounts for 1/3 of the four fruits.

The planting area of loquat is 1/3 of the total planting area of watermelon, grape and citrus.

Loquat accounts for four fruits 1/4.

Then citrus accounts for1-1/5-1/3-1/4 =13/60 of the four fruits.

The area of citrus is 420* 13/60=9 1 mu.