Let the number of numbers in other set, be N

Then total of numbers in other set = 20N

As average of numbers in 1st set of 4 numbers is 35, the total of its 4 numbers is: 4(35), or 140

Since average of the 2 sets is 30, we can say that: 

30(N + 4) = 20N + 140 ------- Cross-multiplying

30N + 120 = 20N + 140

30N - 20N = 140 - 120

10N = 20

N, or number of numbers in other set = , or  

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