I take it that only 7 stamps are misplaced and that we are asked for the total value of the stamps including those which have been misplaced.
If each is worth $0.15 then, the total worth of 7 stamps is $1.05. Then, we add this value to the value of his remaining stamps, $5.55, the answer would be $6.60.
16 square feet. The cube root of 64 is 4, which is equal to the length of one side. The area of one side is the length times the width, and since this is a cube, the length equals the width, meaning that the side is 4 times 4 which is 16
Y will equal 36. You can figure this out because you know y is double the value of x. If x is 18,you can just multiply it by 2, which equals 36
Repetition is a guaranteed way to learn something. If it is a formula you need to learn, try doing flashcards and reading it over and over again until you can spell it without the flahcard. If it is something like factoring, just do a bunch of factoring problems until you get the feel of it.