Education.com
Try
Brainzy
Try
Plus

Tip #11 to Get a Top ACT Math Score

By McGraw-Hill Professional
Updated on Sep 7, 2011

Almost every ACT has a "multiples" question. We know the recipe for the ACT, one multiples question, one average question, etc. These 50 Skills review exactly what you need. Multiples are great to review; you knew this inside-out back in 5th grade. Mrs. Shortino taught it, you did the 7 minutes of homework, and then went straight to the kickball field. Now, you haven't thought about multiples for 6 years. So, here's a quick review, and guess what, now that you're older, anything that seemed hard in 5th grade will be easy!

Multiples—All the numbers that are divisible by a certain number. Example: The multiples of 3 are 3, 6, 9, 12, 15, 18, 21, etc.

Least common multiple—The lowest number that is a multiple of several numbers. Example: What is the least common multiple of 10, 15, and 20 ?

The trick to finding the least common multiple is to list the multiples of the largest given number and then choose the lowest one that is also a multiple of the other given numbers. This is a great trick; we start with the biggest number because it has fewest multiples to consider and will save time and energy.

So to find the least common multiple of 10, 15, and 20, list the multiples of 20: 20, 40, 60, 80, 100. And choose the first one that is also a multiple for both 10 and 15. The number 60 works because it is the lowest number on the list that is a multiple of 10, 15, and 20.

Of course, for most questions, you'll also just be able to "Use the Answers," which will probably be even faster. Note: You can use this same strategy when an ACT question asks for the lowest common denominator; it's basically the same thing.

Let's look at this question:

Solution: This question could be difficult, but "Use the Answers" makes it easy! Just test each answer choice by dividing it by 30, 40, and 50. The number 600 is the lowest number on the list that is divisible by 30, 40, and 50. Notice that 1200 works also, but 600 is the lowest. Make sure to try all choices.