Well, if it is increasing at a fixed rate of 2 mm/s, then the volume would still be increasing at a rate of 2 mm/s when the diameter is 80 mm, if I understand your question correctly.
IF MARTHA EARNS 3.25/HOUR FRO BABYSITTING (SHE IS UNDERPAID!) IN 1.5 HOURS=3.25*1.5=4.88 DOLLARS IN 2.5 HOURS=3.25*2.5=8.13 DOLLARS IN 3.5 HOURS=3.25*3.5=11.38 DOLLARS