Josie and the Unicorn

Josie is tired of being left out by her brother just because she's a girl. So when a unicorn appears in her backyard and tells her she is the fearsome warrior destined to save Rainbow Valley, what can she do but offer to help?
But it's going to be harder than she thought. The sprite Amber Evergreen believes only magic combined with maths will save the magical world and Josie hates maths.
Can she use her skill to defeat the Goblin King and still get back home in time for bed?