A coin is so unbalanced that you are likely to get two heads in two successive throws as you are to get tails in one

Let
h = probability of getting heads in a single throw
1 - h = probability of getting tails in a single throw
 

h² = probability of getting 2 heads in 2 successive throws
$h^2 = 1 - h$

$h^2 + h - 1 = 0$

$h = \dfrac{-1 \pm \sqrt{1^2 - 4(1)(-1)}}{2(1)}$

$h = \dfrac{-1 \pm \sqrt{5}}{2}$
 

Use
$h = \dfrac{-1 + \sqrt{5}}{2}$

$h = 0.618$   ←   answer