A Geiger counter registers 480 counts per minute from a radioactive sample placed near it. The background count rate is known to be constant at 30 counts per minute. After 4h, the counter reading (including background) drops to 135 counts per minute. Predict the counter reading, including background, after a further 2h.
a) 66 counts per minute
b) 108 counts per minute
c) 81 counts per minute
d) 57 counts per minute