In ten years, four half-lives have occurred (10/2.5=4). After one half-life, 50% of the original 100% remains. After two half-lives, 50% of the remaining 50% remains (so 25% remains).
After three half-lives, 50% of the 25% remains, so 12.5% remains.
After four half-lives, 50% of the 12.5% remains, so 6.25% remains.
After four half-lives, 6.25% of the original mass remains unchanged. In fraction form that is 6.25/100
