The exact amount is 10.899967726 % so you're close enough bobbin :)
My working:
20000 x I^8 = 45758.55
I^8 = 45758.55 / 20000
Taking the 8th root of this, gives I = 1.1089967726
to work out %:
(1.1089967726 - 1) x 100
Interest rate compounded every 6 months for 4 years = 10.89967726 %