It is suitable to start with a "let ..." statement defining the meaning of the variable(s) you use in the problem. Here, problems 1, 2, 4 have you multiply the variable by some constant representing a unit rate. In problem 3, you additionally have to perform some math to find an amount remaining.
1) Let s represent the number of shirts purchased at 24.95 each. Then the total cost in dollars is ...
... 24.95s
2) Let t represent the number of trees watered. Then the total number of gallons of water used is ...
... 30t
3) Let q represent the quantity of marbles given away. Then the number remaining is ...
... 48 - q
4) Let p represent the number of pairs of pants purchased at 32.95 each. Then the total cost in dollars is
... 32.95p