Answer: 16. 21ft
17. 116.67mi or ≈117mi
18. 1,377,795in or 3,499.993m or ≈3500m
How we would calculate each of these: For number 16, you would set up a proportion of 1/3=7/x and then simplify this to 1x=21 which results in you dividing 1 from both sides of the equation and getting 21 as x, or the amount of feet. For number 17, you would do the same thing except with the different numbers. You set up another proportion. 3/50=7/x and then simplify to 3x=350. Divide the 3 on both sides and you get x=116.67mi to which you can round to 117mi if you like. For number 18, you must convert the 3.5 kilometers to inches and get 1,377,795in and then you can convert those inches into meters which would be 3,499.993m or you can round it to around 3500m.