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This problem trains for: SAT-I, AMC-8, GMAT.

Mollie and Freddie share a car. Freddie uses it 80% of the time. Freddie's usage is 90% on week-days, while Mollie uses the car 80% on week-ends. What percentage of the car usage happens on week-ends?

Assume the usage of the car is 100 units per 7-day week. Of these units, Freddie uses 80 and Mollie uses 20.

Of the 80 units, Freddie uses:

on week-days and the remaining 8 units on week-ends.

Of the 20 units, Mollie uses:

on week-days and the remaining 16 units on week-ends.

The total number of 'units' used on week-ends by either Freddie or Mollie is:

and this is out of an assumed total number of 100 units. Therefore, the car is used 24% on week-ends.