Dana would go 18 miles in 90 minutes because she is going a mile for every five minutes, so if she bikes for another forty minutes she will have biked 8 miles. This means that you would multiply 8 miles (remember its a mile per 5 minutes) by five minutes and that would give you forty. Then just add that on to the 50 minutes she has already biked.
or do it this way
To get the answer you need to find out how many miles she bikes in 10minutes.
Divide 50minutes by 10 you get 5. Now you divide 10(miles) by 5 and you get 2. So it means that in 10minutes she bikes 2miles. Now you need to know how many miles she bikes in 90minutes so if she bikes 2 in 10 then because 90:10=9 you do 2*9=18. She can bike 18miles in 90minutes. :))
