Giải Bài 3 trang 31 SGK Toán 5

Lời giải

Nhắc lại quan hệ giữa các đơn vị:

$1\text{ cm}=10\text{ mm}$; $1\text{ m}=100\text{ cm}$; $1\text{ km}=1000\text{ m}$; $1\text{ dm}=10\text{ cm}$; $1\text{ kg}=1000\text{ g}$; $1\text{ tấn}=1000\text{ kg}$.

a)

Vì $1\text{ cm}=10\text{ mm}$ nên $1\text{ mm}=\dfrac{1}{10}\text{ cm}$, do đó $3\text{ mm}=\dfrac{3}{10}\text{ cm}$.

Vì $1\text{ m}=100\text{ cm}$ nên $1\text{ cm}=\dfrac{1}{100}\text{ m}$, do đó $6\text{ cm}=\dfrac{6}{100}\text{ m}$ và $52\text{ cm}=\dfrac{52}{100}\text{ m}$.

Vì $1\text{ km}=1000\text{ m}$ nên $1\text{ m}=\dfrac{1}{1000}\text{ km}$, do đó $750\text{ m}=\dfrac{750}{1000}\text{ km}$.

b)

Vì $1\text{ kg}=1000\text{ g}$ nên $1\text{ g}=\dfrac{1}{1000}\text{ kg}$, do đó:

$5\text{ g}=\dfrac{5}{1000}\text{ kg}$; $\quad 30\text{ g}=\dfrac{30}{1000}\text{ kg}$; $\quad 274\text{ g}=\dfrac{274}{1000}\text{ kg}$.

Vì $1\text{ tấn}=1000\text{ kg}$ nên $1\text{ kg}=\dfrac{1}{1000}\text{ tấn}$, do đó $963\text{ kg}=\dfrac{963}{1000}\text{ tấn}$.

c)

Vì $7\text{ cm}=\dfrac{7}{10}\text{ dm}$ nên $2\text{ dm }7\text{ cm}=2\dfrac{7}{10}\text{ dm}$.

Vì $35\text{ cm}=\dfrac{35}{100}\text{ m}$ nên $4\text{ m }35\text{ cm}=4\dfrac{35}{100}\text{ m}$.

Vì $680\text{ g}=\dfrac{680}{1000}\text{ kg}$ nên $5\text{ kg }680\text{ g}=5\dfrac{680}{1000}\text{ kg}$.

Vì $78\text{ kg}=\dfrac{78}{1000}\text{ tấn}$ nên $1\text{ tấn }78\text{ kg}=1\dfrac{78}{1000}\text{ tấn}$.

Đáp số