lichenometry

people use the diameter or the rate of growth of lichen as a means of dating historical events

formulas have been devised to enable dates to be estimated

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 question 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'?