Investment now = £60,000.
Cash return at the end of year 10 = £120,000.

Calculate the Internal Rate of Return

Solve for r.

Oooo!
How do I do that!

4 steps...

• Divide both sides by 60K
• Take the 10th root of both sides (you'll need a calculator which handles fractional roots, or Excel, or something similar)
• That leaves you with (1 + r) on the RHS, so deduct "1" from both sides
• Call it a day

Alternatively, just keep hitting the right-hand side of my initial equation with different values for r, trial-and-error style, until the result is close enough to 120K.