We use the formula of the half-life to calculate for the remaining U-238 after 4.5 billion years. The formula is expressed as A = A₀ (1/2)^(t/h) where A is the final amount, A₀ is the initial amount of the substance, t is the time and h is the half-life of the substance wherein for U-238 h is equal to 4.47 billion years.
A = A₀ (1/2)^(t/h)
A = 50 (1/2)^(4.5 / 4.47)
A = 24.88 g