If you can calculate a 10 percent tip in your head easy then you can calculate both a 20 percent tip and a 15 percent tip. Note that in the case of 1200 minus 375 the complement of 625 might pop up and give you 200 625 825 but a competing method of 800 1200 minus 400 plus 25 825 might arise to you first. Calculate the result in your head by adding the first and last numbers of the number to be multiplied by 11 together and putting that result in the middle.
Back them up with references or personal experience.
Your annual salary is your hourly rate times the hours you work in a week times 52 weeks. Mind Your Puzzles is a collection of the three Math Puzzles books volumes 1 2 and 3. Info on basic mathematics. Making statements based on opinion.