Let's see what happens if we used different values instead of 0.5 for k in the formula S(t) = 10(k)^t÷29

S(t) = 10(1)^t÷29

S(t) = 10 × 1

S(t) = 10

1 raised to any power is still 1. So regardless of t, the original amount of strontium will be multiplied by 1, I.e. remain as 10 forever.

S(t) = 10(0)^t÷29

S(t) = 10 × 0

S(t) = 0

We see from the above that k represents the proportion of strontium that remains after 29 years. When k = 1, 100% of the strontium remains. When k = 0, 0% of the strontium remains.

It just so happened that in this question k = 0.5 means 0.5 of the strontium remains and 0.5 of it remains (both are 0.5). But if k was let's say 0.7 then 0.7 remains and 0.3 decays (both are different) hence the confusion.