Please help me I am stuck.

You need to know the half life of uranium-235.

Then you use the formula:

N = N_o \ \times\ \(\frac12\)^{(\frac{t}{T})}

Here, N is the amount at a certain year
No is the initial amount at year '0'
t is the number of years, here 1 420 000 000 years
T is the half life of the Uranium. (Wikipedia gives it as 704 million years)

Let your initial amount be 100 units. Find the number of units remaining after 1420 million years using the equation.

Then you take you answer, divide it by 100 (the initial amount) and multiply by 100%. Or if you already understood why I took 100, it's because the answer gives directly the percentage of Uranium left.