It was at Monday 17 … DAD sends this solution:

“No 17 Radius = 5.

Proof is too long to print here, but we can confirm the answer from the Small Triangle 4, (8-r) and r. 4squared + (8-r) squared = r squared. 

If r=5 then the sum of [4squared + (8-r)] squared = 16 + 9 = 25 Therefore r = the square root of 25; which = 5 QED.

IE we have the 3:4;5 right angle triangle.”

……

JH: Wunnerful. Wonder if this might also help: