What was the monthly subscription fee for the "Standard" tariff in 2020 if it increased by 50% compared to 2019?

Let's break this down step by step!

1. Find the 2019 monthly fee: The problem states that the monthly subscription fee for the "Standard" tariff in 2019 was 350 rubles.
2. Calculate the increase amount: The fee increased by 50%. To find 50% of 350 rubles, we can calculate:
   50% of 350 = (50/100) * 350 = 0.5 * 350 = 175 rubles.
3. Calculate the 2020 monthly fee: Add the increase amount to the 2019 fee:
   350 rubles (2019 fee) + 175 rubles (increase) = 525 rubles.

So, the monthly subscription fee for the "Standard" tariff in 2020 was 525 rubles.

Ответ: 525 руб.