Give an example of a common “parent-daughter” combination.
the passages is downs below
A common "parent-daughter" combination that geologists use is radioactive uranium and non-radioactive lead. As shown in the diagram above, uranium is trapped in a newly formed rock. As the rock ages, more and more of the uranium changes into lead.

The age of the rock in years can be found by measuring the rate at which a parent element decays and then measuring the ratio of parent element to daughter element in the rock. The ages in years of the different geological time periods are found by measuring the absolute ages of many rocks from all of the different periods. The absolute ages of some of the different geologic time periods are shown along the right side of the Staircase of Time.

The steps of the Staircase of Time are drawn to be almost the same size, so you might think that the time periods are the same length, but they are not. The absolute ages of rocks taken from the different time periods have shown that the time periods were of greatly differing lengths. Some were very short, like the Quaternary period (only 2 million years), while others were very long, like the Proterozoic Era (almost 2 billion years). According to absolute-age measurements, an accurate representation of the lengths of the major geologic time periods is shown in the time bar at right.

link: http://www.cotf.edu/ete/modules/msese/earthsysflr/ages.html