Solution: connect OC, let the radius of ⊙O be r, then OC=R, OM=5-R,

Diameter EF⊥CD, vertical foot m, CD=25,

∴CM=DM=5，

In Rt△OMC, it is obtained from Pythagorean theorem: OC2=OM2+CM2.

R2=(5-R)2+(5)2，

The solution is r = 3.

So the answer is 3.