You need to get an equation that relates quantity (in mL) to time t (hours).
Start with a table of values and see if you can find the equation
4 2.5 mL etc
Given a question such as- 'The half life of element 'x' is 2 hours and starting with 10mL of 'x' how much of 'x' is left after t hours?'
What is the best way of approaching this? Draw a graph and find values? Integration?