21. Price increases then demand falls.

p → q

22. If price does not increase then demand does not
fall.

~p → ~q

23. Price increases if and only if the demand falls.

p ↔ q

24. If the diagonals of a parallelogram are
perpendicular then it is a rhombus.

p → q

25. If the parallelogram is not a rhombus then
diagonals of it are not perpendicular.

~p → ~q

26. If the diagonals of a parallelogram are not
perpendicular then it is not a rhombus.

~p → ~q

27. Mango is a fruit but potato is a vegetable.

p ˄ q

28. Either we play football or go for walking.

p ˅ q

29. An angle is a right angle if and only if it is
of measure 90

^{0}.
p ↔ q

30. 14 is a composite number and 15 is a prime number.

p ˄ q