Question

multiply. use a basic fact.

1. 10×3=

so, 10×30=

1. 2×9=

so, 2×90=

1. 2×8=

so, 2×80=

1. 5×5=

so, 5×50=
problem solving
write a multiplication sentence to solve.

1. harlan has 5 antique watches. each watch has a value of $90.

how much are harlan’s watches worth in all?

1. practice keep trying trey uses 40 nails to put up the

frame around each window. there are 5 windows in the
bedroom. how many nails will trey use in the bedroom?

1. chloe uses 80 candy wrappers to make a paper necklace.

she is making necklaces for herself and 9 friends. how many
candy wrappers will chloe need?
vocabulary check

1. circle the number sentence that shows 20 is a multiple of 2.

2×10=20 2+10=12
2×5=10 10÷2=5
test practice

1. which is equal to 52 tens?

a 52,010 c 5,200
b 5,210 d 520

Explanation:

Step1: Calculate basic multiplication

For \(10\times3\), using the basic fact that \(1\times3 = 3\), then \(10\times3=30\).
For \(2\times9\), using the basic fact \(2\times9 = 18\).
For \(2\times8\), using the basic fact \(2\times8=16\).
For \(5\times5\), using the basic fact \(5\times5 = 25\).

Step2: Use the concept of multiplying by tens

If \(10\times3 = 30\), then \(10\times30\) (since \(30=3\times10\)), \(10\times30=(10\times3)\times10 = 30\times10=300\).
If \(2\times9 = 18\), then \(2\times90=(2\times9)\times10=18\times10 = 180\).
If \(2\times8 = 16\), then \(2\times80=(2\times8)\times10=16\times10=160\).
If \(5\times5 = 25\), then \(5\times50=(5\times5)\times10=25\times10 = 250\).

Step3: Solve problem - solving questions

Question9

Number of watches \(n = 5\), value of each watch \(v=\$90\). The total value \(T=n\times v\). Using the basic - fact \(5\times9 = 45\), then \(5\times90=(5\times9)\times10=450\). So the multiplication sentence is \(5\times90 = 450\).

Question10

Nails per window \(n = 40\), number of windows \(w = 5\). Total nails \(N=n\times w\). Using the basic fact \(4\times5 = 20\), then \(40\times5=(4\times5)\times10=200\). So the multiplication sentence is \(40\times5=200\).

Question11

Number of people \(p=1 + 9=10\), candy - wrappers per necklace \(c = 80\). Total candy - wrappers \(C=p\times c\). Using the basic fact \(8\times10 = 80\), then \(80\times10=(8\times10)\times10 = 800\). So the multiplication sentence is \(80\times10=800\).

Step4: Vocabulary check

A multiple of a number \(a\) is a number \(b\) such that \(b=a\times k\) for some integer \(k\). For \(20\) and \(a = 2\), if \(k = 10\), then \(2\times10=20\). So we circle \(2\times10 = 20\).

Step5: Test practice

52 tens means \(52\times10\). Using the basic fact \(52\times1=52\), then \(52\times10=(52\times1)\times10 = 520\).

Answer:

1. \(10\times3 = 30\), \(10\times30=300\); 6. \(2\times9 = 18\), \(2\times90 = 180\); 7. \(2\times8=16\), \(2\times80 = 160\); 8. \(5\times5 = 25\), \(5\times50=250\); 9. \(5\times90 = 450\); 10. \(40\times5=200\); 11. \(80\times10=800\); 12. Circle \(2\times10 = 20\); 13. D. \(520\)