[tex] \frac{1}{x2} - \frac{1}{x1} = \frac{1}{f} [/tex]
[tex] \frac{1}{x2} + \frac{1}{40} = \frac{1}{25} [/tex]
[tex] \frac{40 + x2}{40 \times x2} = \frac{1}{25} [/tex]
[tex]25 \times (40 + x2) = 40 \times x2[/tex]
[tex]1000 + 25x2 = 40x2[/tex]
[tex]1000 = 40x2 - 25x2 = 15x2[/tex]
[tex]x2 = \frac{1000}{15} [/tex]
[tex]x2 = 66.66[/tex]
[tex]d = x1 + x2[/tex]
d=40+66.66=106.66