Answer:
23.2%
Step-by-step explanation:
First we need to find how many half lives 135 years is:
135 ÷ 64 = 2.109375
Now we divide 100% by 2 to the power of the number of half lives. This is how you calculate the percentage of the original amount left. If you have the original mass, you can substitute that in instead of 100%:
100 ÷ = 23.1747390635
To 1 decimal place it is 23.2%