I know that many of you have been waiting patiently for some more mental math tricks to amaze friends and family (for my previous post go here). With the holidays coming up, I want to be sure that you have some time to practice these new techniques.
Today we’ll look at how to square two-digit numbers that end in “5.” This is actually quite easy. You only need to remember two things:
1. The beginning part of the answer is found by multiplying the first digit by the next higher digit.
2. The answer will end in 25.
I’ll continue to parcel out mental math morsels, but if you really can’t wait, this is a good resource. My mother taught me most of the mental math tricks that I know and this book covers all of them, plus more.
I also promised to keep feeding you some of the problems from Milne’s Mental Arithmetic (1897). So here are a few more. The category is Addition.
A merchant deposited in a bank $25 more on Tuesday than on Monday. If Monday’s deposit was $65, what was the amount of both deposits?
Twenty-five minutes ago it was a quarter to ten. What time will it be one hour and 10 minutes hence?
A farmer sold 27 sheep, lost 16, and had 84 left. How many had he at first?
And one easy challenge (remember, this is supposed to be mental math…don’t pick up a pen/pencil/crayon/marker/quill). This is actually the first (therefore, easiest) of 123 “miscellaneous problems” at the end of Milne’s Mental Arithmetic.
If 1/2 of A’s money is increased by 2/3 of his money, the sum is $560. How much has he?
Curious about #123? Here it is.
A merchant’s expenses during a certain year were 33 1/3% of the gross gain on his sales. His goods were marked 30% above cost. At the beginning of the next year he advanced the price of his goods 10% of the marked price, and during the year cut down his expenses 10%. What per cent did he gain during the second year?