Number Sense Blog

Let us show you a secret: percent means something.   Per means for each, and cent means hundred.   If we want to find 5% of 300, we need to find “5 for each hundred”. So, we count, 5 for the first hundred, 5 for the second hundred, ...

Do you remember our secret from our first video? Percent means “for each hundred”. If we want to find 12% of 250, we need to find “12 for each hundred“. In this case, we have two hundreds and half of one hundred. So, we count, 12 ...

The word half should bring to mind the image of two equal parts: “2 parts the same.”   To find half of any amount, cut the whole into “2 parts the same” and take either one. To find half of 6, we cut the whole of 6 circles into “2 parts t...

Recall that half means “2 parts the same,” “two equal parts.”   Half of 6 circles is 3 because we cut the whole into “2 parts the same” and take one part.   To find half of 7, we bring in one more circle and cut our new whole into “2 p...

The word quarter means “4 parts the same,” “four equal parts”.   To find a quarter of any amount, cut the whole into “4 parts the same” and take any one part. The trick to doing this easily is to “cut it in half and then cut the half in half...

Here is a number sense trick to help you calculate addition using mental math.   We want to find the sum of 9 + 8 + 7. Using our number sense, we turn to “friendly numbers” such as 10 or multiples of 10, to help make this math easier to work...

Here is a number sense trick to help you calculate subtraction using mental math.   We want to find the difference of 100 – 76. Using our number sense, we turn to “friendly numbers” to calculate how far apart are 76 and 100.   Starting...

Here is a number sense trick to help you calculate multiplication using mental math.   We want to find the product of 5 x 4 x 3 x 2 x 1. Using our number sense, we want to find numbers close to ten or multiples of ten to make the multiplicat...

“Special percents” are easy to compute because they represent easy-to-calculate amounts.   0% means “none of it” 25% means “a quarter of it” 50% means “half of it” 100% means “all of it” 200% means “all of it, twice”   To f...

Recall that 10% means 10 for each 100, 10 out of 100, or ten one-hundredths.   We can divide 1 by 10 to convert 10% to a decimal. 10% is equal to 0.1. Notice that dividing by ten moves the decimal point one place to the left.   To find...

Recall that 1% means 1 for each 100, 1 out of 100, or one one-hundredth.   We can divide 1 by 100 to convert 1% to a decimal. 1% is equal to 0.01. Notice that dividing by one hundred moves the decimal point two places to the left.   To...

A number is divisible by 3 if the sum of the digits is divisible by 3.   To determine if the number 87 is divisible by 3, we find the sum of its digits. Eight plus seven gives us a final sum of fifteen. Fifteen is divisible by 3. So,   ...

One way to test if a three-digit number is divisible by 11 is to follow the 3-digit rule: the sum of the first and last digits equals the middle digit.   To determine if the number 792 is divisible by 11, we find the sum of seven and two. Se...

To test if a number is divisible by 11, follow the general rule: the difference of the sum of alternating digits can be divided by 11.   To determine if the number 2,728 is divisible by 11, find the difference of the sum of alternating digit...

To find half of even numbers, sometimes it is useful to break apart the whole number into its parts. We can do this to find half of 26. Since 26 is composed of 2 tens and 6 ones, we can find half of 26 by finding half of 20 and half of 6. Half ...

Here is a number sense trick to subtract without borrowing. Let’s say we want to subtract 54 from 900. Since the digits in the ones and tens places of 900 are both less than the same digits in 54, we need to borrow to find the difference. Wi...

A number is divisible by 9 if the sum of the digits is divisible by 9.   To determine if the number 738 is divisible by 9, we find the sum of its digits. Seven plus three plus eight gives us a final sum of eighteen. Eighteen is divisible by ...

Welcome to Mathnasium’s Math Tricks! Here’s a trick to calculate percentages using the commutative property of multiplication. Let’s do this! 👍   To find 48% of 50 means to multiply 48 x  x 50. We can use the commutative property of multi...

  Welcome to Mathnasium’s Math Tricks series. Our previous Math Tricks have looked at strategies for finding half of odd numbers whose tens digits are even. Today we are focusing on a strategy to find the half of any odd number, such as 99… or ...

  Welcome to Mathnasium’s Math Tricks series. When kids learn their multiplication facts, they often first master their five times tables. But multiplying large numbers by five can get tricky. Today we are using the fact that five is half of te...