1. **Problem statement:** The mass of a radioactive substance decreases exponentially at a rate of 3% each day. We need to find the overall percentage decrease after 10 days. 2. **Formula used:** For exponential decay, the amount remaining after $t$ days is given by: $$ M(t) = M_0 (1 - r)^t $$ where $M_0$ is the initial mass, $r$ is the decay rate per day (as a decimal), and $t$ is the time in days. 3. **Given values:** - Decay rate $r = 3\% = 0.03$ - Time $t = 10$ days 4. **Calculate remaining mass fraction:** $$ M(10) = M_0 (1 - 0.03)^{10} = M_0 (0.97)^{10} $$ 5. **Evaluate $(0.97)^{10}$:** $$ (0.97)^{10} \approx 0.7374 $$ 6. **Interpretation:** After 10 days, approximately 73.74% of the original mass remains. 7. **Calculate overall percentage decrease:** $$ \text{Decrease} = 100\% - 73.74\% = 26.26\% $$ **Final answer:** The overall percentage decrease after 10 days is approximately **26.26%**.