people use the diameter or the rate of growth of lichen as a means of dating historical events
simpler formulas assume a circular growth pattern
a Pisa test item (international test, M047) provided a formula used in studies of glaciers
d is the diameter of the 'circle' in mm.
t is the number of years after ice has disappeared
the test asked students to use the formula to find an estimate for a diameter after 16 years
another question might have been:
for what two values of 't' does this formula predict the same numerical value for 'd' as for 't'?