First, we must recognize that all pairs of shoes and socks are actually two items each, whereas the pants and the t-shirts are individual items. Therefore there are 20*2= <<20*2=40>>40 individual socks in 20 pairs of socks.
Similarly, there are 5*2=<<5*2=10>>10 individual shoes in 5 pairs of shoes.
Therefore, Jonas has 40+10+10+10= <<40+10+10+10=70>>70 individual items in his collection.
To double this number, we perform 70*2=<<70*2=140>>140.
This means we need 140-70=<<140-70=70>>70 new items to double the size.
Since there are 2 individual socks in each pair, we take 70/2= <<70/2=35>>35 new pairs of socks.
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