From the isometric box:

$s = \sqrt{x^2 + 30^2}$

$s = \sqrt{x^2 + 900}$

where:

x^{2} = (20t)^{2} + (40 - 30t)^{2}

x^{2} = 400t^{2} + 1600 - 2400t + 900t^{2}

x^{2} = 1300t^{2} - 2400t + 1600

$s = \sqrt{(1300t^2 - 2400t + 1600) + 900}$

$s = \sqrt{1300t^2 - 2400t + 2500}$

$\dfrac{ds}{dt} = \dfrac{2600t - 2400}{2\sqrt{1300t^2 - 2400t + 2500}}$

$\dfrac{ds}{dt} = \dfrac{1300t - 1200}{\sqrt{1300t^2 - 2400t + 2500}}$

after 1 sec, t = 1

$\dfrac{ds}{dt} = \dfrac{1300(1) - 1200}{\sqrt{1300(1^2) - 2400(1) + 2500}}$

$\dfrac{ds}{dt} = 2.67 \, \text{ ft/sec}$ *answer*