Answer
$4.2 $ feet per second
Work Step by Step
distance = (rate)$\times$(time) $\Rightarrow$ time = $\displaystyle \frac{\text{distance}}{\text{rate}}$
Let x be the walking speed (rate).
We are given:
$\displaystyle \frac{100}{x+1.8}=\frac{40}{x-1.8} \qquad$ ... $/\times(x+1.8)(x-1.8)$
$100(x-1.8)=40(x+1.8)$
$ 100x-180=40x+72\qquad$ ... add $180- 40x$
$ 60x=252\qquad$ ...$ /\div 60$
$x=4.2 $ feet per second
walking speed = $4.2 $ feet per second