Hi,

If you drive 40 feet in one tenth of a second then in ten times that long (1 second) you travel ten ties as far(400 feet). Thus you are driving at the rate of 400 feet per second.

This is a perfectly good answer but I expect you want it expressed in miles per hour. Let's do it in steps, first converting seconds to minutes. There are 60 seconds in a minute so in 60 times 1 second you travel 60 times 400 feet. Hence you are travelling at $60 \times 40$ feet per minute.

How many minutes are in an hour? What is your speed in feet per hour?

How many feet are in a mile? What is your speed in miles per hour?

Penny