For number 72, 15 more than a number means +15 of some number "x". Here x can be any number, but calling it x is simple enough to represent a whole spectrum or range of values x can truly be. Keyword is more than which usually hints it's 'an addition to'.
So x+15 is the correct expression given that 'x' is that number.
For 73,
What is the variable for the number of starfish? Is that given to you? I'll assume not, so we define one. Let 's' represent the number of starfishes. The keyword here is 'less than' which hints it's a subtraction problem. 9 less than means minus 9 of some value, and here that value is the number of starfishes which is 's'.
So it would be s-9
For 74, half the number of something means (1/2) of that value. If you have (1/2) of the number of pencils, then define a variable first for the number of pencils. Let 'p' be the number of pencils. Then,
correct answer is (1/2)*p or 0.5p. Keyword is "of" which could be an good indicatior of mutilplication.
For 75, keyword is "increased by", which is a indicator of addition. If you let the number of chairs be defined by "c", then the correct answer is c+7