Ineed with this too why is math so hard ?

Question 7;
20 mg iodine is given. Since the half life of it 8 days,
It will reduce to 10mg after 8 days. ( 20/2 =10mg)
It will reduce to 5mg after another 8 days. (10/2 =5mg)
It will reduce to 2.5mg after another 8 days.(5/2 =2.5mg)
And it will reduce to 1.25mg after another 8 days.

So after 32 days which is 4 * 8 days, it will be only 1.25mg left.

Question 8a ;
Growth factor is 3 because you multiply every number to get next one. 3*3 =9 , 9*3=27 , 27*3=81
Growth rate is %300 because it is 3 times of the actual number. %100 of number 3 is 3, %200 of number 3 is 6 and %300 of number 3 is 9.

Question 8b;
Model is the first one, y=3^x
Because sequence is orders of 3. 3 order of 1 is 3, 3 order of 2 is 9 , 3 order of 3 is 27.