What is the algebraic representation of 3 times the difference of a number and 5

Two variables, x and y, vary inversely, and x=12 when y=4. what is the value of y when x=3?

He mass of a single atom of carbon can be found by dividing the atomic mass (12.01 g) by 6.022 x 10^23. which is the mass of a single carbon atom, correctly written in scientific notation with the correct number of significant figures?

Twelve less than a number is at most 20