Question

Plant A grows 5 cm in 7 days and plant B grows 4 cm in 6 days. The difference in their growth rates is cm per day.

• A. $\frac{1}{7}$
• B. $\frac{1}{42}$
• C. $\frac{1}{21}$
• D. $\frac{1}{6}$

Answer 1

The correct option is C $\frac{1}{21}$

$\text{Plant A grows 5cm in 7 days.}$

$⟹\text{Growth rate of plant A}=\frac{5}{7}\text{cm per day}$

$=\frac{5\times 3}{7\times 3}\text{cm per day}$

$=\frac{15}{21}\text{cm per day}$

$\text{Plant B grows 4cm in 6 days.}$

$⟹\text{Growth rate of plant B}=\frac{4}{6}\text{cm per day}$

$=\frac{4÷2}{6÷2}\text{cm per day}$

$=\frac{2}{3}\text{cm per day}$

$=\frac{2\times 7}{3\times 7}\text{cm per day}$

$=\frac{14}{21}\text{cm per day}$

$\therefore \text{Difference between the growth rates}$ $\text{of the two plants}$

$=(\frac{15}{21}-\frac{14}{21})\text{cm per day}$

$=\frac{1}{21}\text{cm per day}$