Are you looking to skyrocket your calculation speed for high stakes exams? Whether you are preparing for the West African Examinations Council (WAEC), NECO, JAMB, SAT, or competitive civil service tests, managing your time efficiently is the ultimate key to scoring an A1. Here is the guide you need. A lot of students in school are still having these problems, but the Simplemethd11 Series will show you exactly what to do when asked."
In our previous guide, we broke down the foundational hacks for how to multiply any two digit number by 11. In Simplemethd11 Series we are giving you a good mental brain work in further. We will explore advanced algebraic decomposition tricks that allow you to rapidly multiply any two digit number by 12, 13, 14, and 15 cleanly inside your head in 5 seconds or less.
The Core Principle: Master Mental Distribution
The scope behind all the ways you can apply mental Mathematics shortcuts formation is to focus and read your note. Instead of looking at 12, 13, 14, or 15 as scary static blocks, your brain must learn to split them into a highly manageable base of ten plus a small single digit variable. This convert an external ways of multiplication to solve problems and convert it back into simplemethd11 of addition of two partial products.
To multiply any number by 12, multiply the base number by 10 (just append a zero), double the original base number, and add the two sums together.
Formula: (N × 10) + (N × 2)Worked Example: Calculate 34 × 12 mentally.
- Step 1: Multiply 34 by 10 = 340
- Step 2: Double 34 (34 × 2) = 68
- Step 3: Sum the results (340 + 68) = 408
Multiplying by 13 follows the exact same logical framework, but instead of doubling your base value, you add its triple product to the baseline factor.
Formula: (N × 10) + (N × 3)Worked Example: Calculate 15 × 13 mentally.
- Step 1: Multiply 15 by 10 = 150
- Step 2: Triple 15 (15 × 3) = 45
- Step 3: Sum the results (150 + 45) = 195
Multiplying by 14: The "Double-Double" Addition
Multiplying by 14 requires adding 4 times the base number to its base-10 value. To find 4 times a number instantly without thinking, simply double it twice.
Formula: (N × 10) + (Double-Double N)Worked Example: Calculate 21 × 14 mentally.
- Step 1: Multiply 21 by 10 = 210
- Step 2: Double 21 to get 42, then double 42 to get 84 (21 × 4) = 84
- Step 3: Sum the results (210 + 84) = 294
Multiplying by 15: The Elegant "Half Add" Trick
Multiplying by 15 is unique because 15 is exactly 1.5 times 10. This means you can find the product by taking the base-10 value, finding exactly half of it, and adding that half back to the whole value.
Formula: (N × 10) + [(N × 10) / 2]Worked Example: Calculate 24 × 15 mentally.
- Step 1: Multiply 24 by 10 = 240
- Step 2: Find half of 240 = 120
- Step 3: Add them together (240 + 120) = 360
Speed Drill: 12 Practice Questions & Detailed Answers
Put your newly learned mental math skills to work immediately. Try to solve these questions purely in your head before checking the structured answers below.
Question 8: Find the value of 42 × 14.
Answer: 588
Mental Math Steps: 42 × 10 = 420. (42 doubled is 84, then 168 . Adding them gives 420 + 168 = 588.
Question 9: Solve 18 × 14 mentally.
Answer: 252
Mental Math Steps: 18 × 10 = 180. 18 doubled is 36, then 72). Adding them gives 180 + 72 = 252.
Question 10: What is 36 × 15?
Answer: 540
Mental Math Steps: 36 × 10 = 360. Half of 360 is 180. Adding them gives 360 + 180 = 540.
Question 11: Calculate 54 × 15 using the half-and-add rule.
Answer: 810
Mental Math Steps: 54 × 10 = 540. Half of 540 is 270. Adding them gives 540 + 270 = 810.
Question 12: Find the product of 82 × 15.
Answer: 1230
Mental Math Steps: 82 × 10 = 820. Half of 820 is 410. Adding them gives 820 + 410 = 1230.
Rate Your Mental Speed
Time yourself on how fast you answered the 12 drill questions above without writing anything down, then match your results below:
Average Time Per Question
Skill Ranking Status
Under 4 Seconds
🚀 Elite Math Wizard (Exam Ready)
5 - 10 Seconds
🏃 Steady Competitor (Solid Progress)
Over 10 Seconds
📚 Aspiring Learner (Needs Daily Drills)
Conclusion:
Bookmark this page to practice these drills, share this post with your classmates preparing for upcoming exams, and tell us in the comment box below which shortcut helped you speed up your calculation times the most.
Master Mental Math for Exam Success: Boost your calculation speed for WAEC, JAMB, SAT, and other competitive exams with these Brainimatic techniques. Following up on multiplying by 11, this guide covers fast, mental methods for multiplying two-digit numbers by 12, 13, 14, and 15.
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