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The initial count rate is 150 counts/s. Half of this is 75 counts/s. From the table, the count rate reaches 75 counts/s at t = 180 s. However, the count rate at t=120 s is 95, and at t=180 s is 75. The time to drop from 150 to 75 is approximately between 120 and 180 seconds. More precisely, the half-life is when the count rate is 75. Looking at the table, 75 is at 180 s. But let's check: 150 -> 75 is one half-life. 75 -> 37.5 would be another. The table doesn't go that far. The count rate drops to 75 at 180 s. So the half-life is 180 seconds? Let's re-read the data: time: 0,60,120,180,240; count: 150,120,95,75,60. The count rate drops from 150 to 75 (half) between 120 and 180 seconds. The exact half-life is when it is 75, which is at 180 seconds. But the options are 60, 120, 180, 240. 180 is correct.