It is easier to write the equivalent ratios first, then use that information to find the missing values.
3. ratio: cars/trucks = 3/5, or trucks/cars = 5/3. Missing value: 6/3*5 = 10.
4. ratio: TVs/Computers = 2/7, or Computers/TVs = 7/2. Missing value: 21/7*2 = 6.