20°C less than the present temperature.
t - 20
Step 1: Let the present temperature be \(t\) (in °C).
Step 2: “20°C less than” means we subtract 20 from the present temperature.
Write it as:
\( ext{New temperature} = t - 20 \)
Step 3 (with units):
\( t^{\circ}mathrm{C} - 20^{\circ}mathrm{C} = (t - 20)^{\circ}mathrm{C} \)
Therefore: the expression is \( t - 20 \).