Answer:
27 cm
Step-by-step explanation:
The right triangle shown in the diagram can be used to find the radius r. By the Pythagorean theorem, ...
24² + (72-r)² = (24+r)²
576 + 5184 -144r +r² = 576 +48r +r²
5184 = 192r
5184/192 = r = 27 . . . . cm
The radius of the third circle is 27 cm.