Rhombus

The formula for the area of a rhombus is $A=\frac{pq}{2}$  A representing the area, and p and q as the two diagonal lengths

You can plug in the area and one of the diagonals to get $20=\frac{10q}{2}$

Now you just solve for q. 40 = 10q, q=4

The length of the other diagonal is 4