Smiths have driven 400 miles which 5/8 of their trip. How much further do they have to drive

First, let's translate this into an equation.

400 is 5/8 of what number?

The missing number in this equation is the total trip miles.

400 x 8 is 3200

3200 divided by 5 is 640

640 is the total trip miles. Now we can take 3/8 of the 640 or we can just subtract 400 from 640.

640 minus 400 is 240. Let't check our answer.

640 times 3 is 1920. 1920 divided by 8 is 240.

How much further do they have to drive? The have to drive 240 more miles.