We need to know the equivalence between centimeters and milimeters to solve this problem.
1 cm = 10 mm
So we can use a rule of three simple and with direct proportion to solve it:
10 mm -----> 1 cm
2 mm ------> x
x = (2)(1)/10
x = 0.2 cm
therefore there are 0.2 cm in 2 mm