A weather balloon calibrated at 0.00 °C to have a volume of 20.0 L has

what volume at - 40.0 °C assuming pressure is held constant?

17.1 Liters

Explanation:

It's a gas law question (more specifically a Charles's Law question). Formula is V1/T1 = V2/T2. You're given V1 and T1 and T2. However, in order to use the equation, temperature needs to be in Kelvins (by subtracting the degrees C from 273) for the numbers to work (among other reasons, the 0 degrees C will always give you an answer of zero or undefined). Placing all temps in kelvins makes the answers come out right. So 20L/273K = xL/233K gives you the answer when you cross-multiply.