First you want to take the miles he drove and figure out how many miles a minute he drove, since there are 60 minutes in one hour we can then do 40 divided by 60 which would give us a unit rate of 0.6666666666666667 miles per minute, you would then multiply this unit number by the second time period which is one hour and 30 minutes; this looks like 0.6666666666666667 times 90, which would equal 60 miles in 1.5 hours... So david drives at a rate of 60 miles in 1.5 hours..