Jennifer Land shares GMAT tips to refresh your multiplication and division knowledge.
Because the GMAT tests math concepts most of us learned in elementary school, it can be challenging to recall some of these basic quantitative building blocks. Boost your ability to do mental math with these tips for remembering how to divide numbers in your head.
Divisibility rules for twos, fives, and tens
The easiest of the divisibility rules are the rules for 2, 5, and 10:
If a number is even, it is divisible by 2.
If a number ends in 0 or 5, it is divisible by 5.
If a number ends in 0, it is divisible by 10.
Divisibility rules for threes, sixes, and nines
If the sum of a number’s digits is divisible by 3, the number is divisible by 3. For example, 432 → 4+3+2=9, so 432 is divisible by 3. But 253 → 2+5+3=10, so 253 is not divisible by 3.
If a number is even AND the sum of its digits is divisible by 3, it is divisible by 6: 432 → 4+3+2=9 and 432 is even (divisible by 2), so it is divisible by 6.
If the sum of a number’s digits is divisible by 9, the number is divisible by 9: 432 → 4+3+2=9, so 432 is divisible by 9; 837 → 8+3+7=18, so 837 is divisible by 9.
An interesting side note about 9: All multiples of 9, when their digits are summed, eventually yield 9. For example, 837 → 8+3+7=18, and 1+8=9.
Divisibility rules for fours
If the last two digits of a number are a multiple of 4, the entire number is a multiple of 4. You don’t add the digits together; if the last two are a multiple of 4 that you recognize, you can trust you have a multiple of 4. Because 100 is divisible by 4, all that matters is the last two digits. For example, 2,348,632 is divisible by 4 because the last two digits, 32, is divisible by 4.
I’m not aware of any useful ways to determine divisibility by 7 or 8, so your best bet for those is to learn their multiples instead.
Multiplication tables
To put these rules into action when prepping for the GMAT, I highly recommend downloading apps designed for 4th graders to learn multiplication. Make it fun! Most of us have forgotten our multiplication tables, so there is no shame in needing to refresh them. Take care to learn the multiples of 13 as well. Because most people brush up on multiples through 12, the GMAT loves to throw in 13 to catch you off guard.
Mastering these basics will allow you to do more mental math on Test Day and will limit the time (and noteboard space) you need to use writing out division and multiplication. Bank those precious seconds for working tougher problems!
Looking to refresh your knowledge of multiplication and divisibility? Explore our free GMAT prep options to get started.
The post Land Your Score: Divisibility Rules for the GMAT appeared first on Business School Insider.
