枚举每个利率

#include <bits/stdc++.h>
using namespace std;
double w0, w, m;
bool check(double p)
{
    double ww0 = w0;
    for (int i = 1; i <= m; i++)
        ww0 = (ww0 * (1 + p) - w);
    return ww0 <= 0;
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> w0 >> w >> m;
    double ans;
    for (double p = 0.0001;; p += 0.0001)
    {
        if (check(p))
            ans = p;
        else
            break;
    }
    cout << fixed << setprecision(1) << ans * 100;
    return 0;
}

二分

#include <bits/stdc++.h>
using namespace std;
double w0, w, m;
bool check(double p)
{
    double ww0 = w0;
    for (int i = 1; i <= m; i++)
        ww0 = (ww0 * (1 + p) - w);
    return ww0 <= 0;
}
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> w0 >> w >> m;
    double l = 0;
    double r = 3;
    while (r - l > 0.0001)
    {
        double mid = (l + r) / 2;
        if (check(mid))
            l = mid;
        else
            r = mid;
    }
    cout << fixed << setprecision(1) << l * 100;
    return 0;
}