`@` `\text {Ans}`
`\downarrow`
`1,`
`a)`
`A = {x \in NN` `|` `16 < x < 21}`
`=> A = {17; 18; 19; 20}`
`b)`
`B = {x \in NN \text {*}` `|` ` x \le 7} ?`
`=> B = {1; 2; 3; 4; 5; 6; 7}`
`c)`
`C = {x \in NN` `|` `13 \le x < 39}`
`=> C = {13; 14; 15; ... ; 38}`