The correct answer is C.
Because: 2.9 multiplied by 10 bags equals 29cubic ft and she needs 30 cubic ft. So she would need to buy 11 bags to make sure she has enough sand for the whole thing. 11 x 2.9 = 31.9 which ensures she has enough for the whole thing including extra in case. So you answer would be Answer C.
Usually when you do expressions involving subtraction you can do Keep, Change ,Change
That means you keep the sign of the first number and change the other two signs.
It is best you rewrite the expression to addition because a lot of people don't do well with subtracting .
Answer:
The equation of line 65 and the sum is 80
