Diana usually drives at an average rate of 30 mph. If Tony's house is 24 minutes away and Diana is able to increase her average rate by 10 mph, how many minutes would it take her to get to Tony's house?
Diana usually drives 30 miles per hour, so in 24 minutes she would have traveled 30 miles per hour * 24 minutes / 60 minutes per hour = <<30*24/60=12>>12 miles.
If Diana increases her average speed by 10 mph, she would be driving at a rate of 30 mph + 10 mph = <<30+10=40>>40 mph.
At a rate of 40 mph, it would take her 12 miles / 40 mph * 60 minutes per hour = <<12/40*60=18>>18 minutes to get to Tony's house. Answer: \boxed{18}.