Help! How to calculate the remaining activity of a radioactive substance

I've got a question on a book about the half-life of a radioactive substance.
The given graph stated that the activity of the radioactive material is 1000 counts per minute at t=0.
It was given that the activity of the particular substance reduced to 800 counts per minute at 20th minute. At the 35th minute, the activity of the substance was further reduced to 400 counts per minute.
The question asked for the remaining activity after an hour.
Given selections are 50, 100, 200 and 400.

My solution is as below,
First, the initial activity of the substance is 1000 counts per minute (cpm). After a half-life, the activity should be reduced to about 500 which appears to be in between the time frame 20 to 35 minutes. Thus, I can deduce that the half-life of the substance should be in between 20 to 35 minutes.
20<Half-life<35
Divided by an hour
3>No. of half-life in an hour>1.7143
Calculating the approximate range of remaining activity should be:
1000 x (1/2)^3< Remaining activity< 1000 x (1/2)^1.7143
125< Remaining activity<304.7504
And thus, the only suitable answer should be 200 cpm as it was the only number in the calculated range.

My teacher's solution is as below,
According to the given value, the half-time of the radioactive substance is 15 minutes as the radioactive activity reduced by half from 20th to 35th minute (800 to 400 as stated above). And thus, starting from t=0, when t=60, 4 half-lives have occurred and the calculation should be:
1000 x (1/2)^4 = 62.5 cpm
The answer is 50 as it is the nearest value to the calculation.
(My reasoning for accusing the solution is incorrect are as below
If the half-life of the substance is 15 minutes, at 20th minute, the activity should be smaller than 500cpm.)

The answer given by the book was 50. There were no solutions or explanation

PS* Can anyone teach me how to attach the image cus it would be much more easier for me to ask questions lol.