Length of the stem at time 0 is 20 cm. The arithmetic growth rate...

The equation for arithmetic growth is given by:

$L_t = L_0 + rt$

Where:
$L_t$ is the length at time $t$
$L_0$ is the initial length at time $t = 0$
$r$ is the arithmetic growth rate
$t$ is the time

Given:
$L_0 = 20$ cm
$r = 30$ cm/day
$t = 7$ days

Substituting the given values into the equation:

$L_7 = 20 + 30 \times 7$

$L_7 = 20 + 210$

$L_7 = 230$ cm