AS OCR Physics Motion Question Help

This is the question
A small hand-held device which emits and detects ultrasonic pulses is used by estate agents as an electronic tape measure for sizing rooms. The device measures the time it takes to receive the echo of an emitted pulse from the far wall. The speed of ultrasound in air is 340m/s. For a particular room the device records times of 20ms and 30ms. Calculate
(i) the floor area of the room
(ii) the mass of air in the room, the height of which is 2.3m. Take the density of air as 1.3kg/m^2

Stuck on the first part as I would of thought that you multiplied 340 by 20x10-3 and 30x10-3 and the multiplied the answers together however in the back of the book the answer only comes to 17.3m^2?

Hi Ben and welcome to TSR.

It's all there, you just need to read the question very carefully:

"The device measures the time it takes to receive the echo of an emitted pulse from the far wall."

What does that underlined part imply for the total distance covered by the pulse?