Дано: $$a = 2,4 \cdot 10^3 = 2400, b = 1,2 \cdot 10^2 = 120$$
$$a + b = 2400 + 120 = 2520$$
$$a - b = 2400 - 120 = 2280$$
$$a \cdot b = 2400 \cdot 120 = 288000$$
$$a : b = \frac{2400}{120} = 20$$
Ответ: $$a+b=2520, a-b=2280, a\cdot b=288000, a:b=20$$