Explanation:
w = t/7
at the beginning we 0 inches of water.
after 56 minutes we have 8 inches of water.
that is a ratio of 56 minutes / 8 inches = 7/1 = 1 / 1/7.
so, in 1 minute we get 1/7 inch of water.
in 2 minutes 2×1/7 = 2/7.
in 3 minutes 3×1/7 = 3/7.
...
hence the equation.