Multiply a number by a digit
1.1. Knowledge to remember
a) 241324 × 2 = ?
Multiply in order from right to left:
 2 times 4 equals 8, write 8.
 2 times 2 equals 4, write 4.
 2 times 3 equals 6, write 6.
 2 times 1 equals 2, write 2.
 2 times 4 equals 8, write 8.
 2 times 2 equals 4, write 4.
\(\begin{array}{l}
\underline { \times \begin{array}{*{20}{c}}
{241324}\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,2}
\end{array}} \\
\,\,\,\,482648
\end{array}\)
241324 × 2 = 482648.
b) 136204 × 4 = ?
Multiply in order from right to left:
 4 times 4 equals 16, write 16 remember 1.
 4 times 0 equals 0, add 1 equals 1, write 1.
 4 times 2 equals 8, write 8.
 4 times 6 equals 24, write 4 remember 2.
 4 times 3 equals 12, add 2 equals 14, write 4 remember 1.
 4 times 1 equals 4, add 1 equals 5, write 5.
\(\begin{array}{l}
\underline { \times \begin{array}{*{20}{c}}
{136204}\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,4}
\end{array}} \\
\,\,\,\,544816
\end{array}\)
136204×4 = 544816.
1.2. Solving Textbook Exercises
Lesson 1: Calculate then calculate
a) 341231 x 2 b) 102426 x 5
214325 x 4 410536 x 3
Solution guide:
 Set the calculation so that the unit digits are aligned, then multiply in order from right to left.
a)
\(\begin{array}{l}
\underline { \times \begin{array}{*{20}{c}}
{341231}\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,2}
\end{array}} \\
\,\,\,\,682462
\end{array}\) \(\begin{array}{l}
\underline { \times \begin{array}{*{20}{c}}
{241325}\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,4}
\end{array}} \\
\,\,\,\,857300
\end{array}\)
b)
\(\begin{array}{l}
\underline { \times \begin{array}{*{20}{c}}
{102426}\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,5}
\end{array}} \\
\,\,\,\,512130
\end{array}\) \(\begin{array}{l}
\underline { \times \begin{array}{*{20}{c}}
{410536}\\
{\,\,\,\,\,\,\,\,\,\,\,\,\,3}
\end{array}} \\
\,\,\,\,1231608
\end{array}\)
Lesson 2: Write the expression value in the empty cell
m 
2 
3 
4 
5 
201634 xm 




Solution guide:
 Replace letters with numbers into the expression 201634 xm and calculate the value of that expression thức
– If m = 2, then 201634 xm = 201634 x 2 = 403268;
– If m = 3, then 201634 xm = 201634 x 3 = 604902;
– If m = 4, then 201634 xm = 201634 x 4 = 806536;
– If m = 5 then 201634 xm = 201634 x 5 = 1008170.
We have the following table of results:
m 
2 
3 
4 
5 
201634 xm 
403268 
604902 
806536 
1008170 
Lesson 3: Calculate
a) 321475+423507×2
843275−123568×5
b) 1306×8+24573
609×9−4845
Solution guide:
 Expressions with addition, subtraction, multiplication, and division operations, we perform multiplication and division first, addition and subtraction later.
a) 321475+423507×2
= 321475+847014 = 1168489
843275−123568×5
= 843275−617840 = 225435
b) 1306×8+24573
= 10448+24573 = 35021
609×9−4845
= 5481−4845 = 636
Lesson 4: A mountainous district has 8 lowland communes and 9 upland communes. Each lowland commune was granted 850 story books, each upland commune was granted 980 story books. How many books are available for that story?
Solution guide:
 Find the number of story books in 8 lowland communes, multiply the number of story books in each lowland commune by 8.
 Find the number of story books in 9 upland communes, multiply the number of story books in each upland commune by 9.
 Find the number of story books that district has been granted, we take the number of story books of 8 lowland communes granted plus the number of story books of 9 upland communes.
Solution
The number of story books distributed to 8 lowland communes is:
850×8 = 6800 (book)
The number of story books distributed to 9 highland communes is:
980×9 = 8820 (book)
The number of story books granted to that district is:
6800+8820 = 15620 (book)
Answer: 15620 books
.
=====
Leave a Reply