A logarithm is an exponent: log₂ 128 = 7 because 2⁷ = 128. Logarithms undo exponentials, so the graph of y = log₂ x is the mirror of y = 2ˣ across the line y = x. To solve a × bᵗ = d, divide by a, then take a logarithm of both sides. The crew uses this to find when the duckweed covers the trough and how long the warm soil takes to cool.