The previous answer is a bit of guess, so does mine, but let us analyse this deeply.

**In Area 2 of Granite Gave (OrAs)**, you can only encounter **one steel type** i.e. *Aron*. It has 30% chance to encounter.
- It will become 50% if your party will be leading with Magnet Pull.
- So the rest of the 'mons have the probabilty of 50% to be encountered. Zubat, Abra and Makuhita are the rest of the 'mons encountered, so the combined availability is 50%.

Without Magnet pull:

- Zubat - 50%
- Aron - 30%
- Abra - 10%
- Makuhita - 10%

*So basically percentage of encountering all Pokemons except Aron is 70%.*

When magnet pull is affected,

- Aron - 50%
- Zubat - 50/70 * 50 = 35.71% (approx)
- Abra - 10/70 * 50 = 7.14%
- Makuhita - 10/70 * 50 = 7.14%

When you add all the values, it will be 99.99 which is approx 100%.

**In Area 3 in Granite Cave**, where **two steel types**, *both Mawile and Aron* are found,

Without Magnet Pull,

- Zubat - 50%
- Aron - 30%
- Abra - 10%
- Mawile - 10%

Possibility of encountering Steel types = 30 + 10 = 40%, non-steel types = 60%

With Magnet Pull, (Aron + Mawile = 50%)

- Aron - 30/40 * 50 = 37.5 %
- Mawile - 10/40 * 50 = 12.5%
- Zubat - 50/60 * 50 = 41.67%
- Abra - 10/60 * 50 = 8.33%

When you add all the values, it will total to 100%

I am not sure in the case where the probability of encountering a steel type is above 50%, whether it reduces to 50% or not. However, it is a guess.

Hope this helps :)