What would the percentage of thorium-232 ( parent compound) be in a rock that was dated at 1.8 billion years

The half-life of Th-232 is 1.405 × 10¹⁰ years  

Time elapsed = 2.8 x 10⁹ years  

Equation of radioactive decay:

A = A₀ = (1/2)^ t/t₁/₂

Thus, the percentage of thorium-232 in the rock that was dated at 2.8 billions year = 87.1%

the atomic mass is the bottom number on the element.

-steel jelly

indicate valleys, hills, steepness and gentleness of slopes.

explanation:

contour lines on a map indicate elevation, or depth. numeric notations on contour lines refer to the level of elevation, or height about sea level.