In a simple Atwood machine, two unequal masses m_1 and m_2 are...

Given, $m_1 = 300 \text{ g} = 0.3 \text{ kg}$ and $m_2 = 600 \text{ g} = 0.6 \text{ kg}$.

Let $T$ be the tension in the string and $a$ be the acceleration of the masses. The equations of motion for the two masses are:
$m_2 g - T = m_2 a$
$T - m_1 g = m_1 a$

Solving these equations for tension $T$, we get:
$T = \dfrac{2 m_1 m_2}{m_1 + m_2} g$

Substituting the given values and $g = 9.8 \text{ m/s}^2$:
$T = \dfrac{2 \times 0.3 \times 0.6}{0.3 + 0.6} \times 9.8$

$T = \dfrac{0.36}{0.9} \times 9.8$

$T = 0.4 \times 9.8 = 3.92 \text{ N}$

Rounding to one decimal place, the tension is approximately $3.9 \text{ N}$.