out of 6 different science books and 4 diff maths books in how many ways can we select 4 books if 2 of each are necessary?

The number of ways of selecting 2 science books out of 6 is 6C2 = 15
The number of ways of selecting 2 maths books out of 4 is 4C2 = 6

Hence, the total number of ways is 15*6 = 90

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