How many A.P.'s with 10 terms are there whose first term is in the set {1,2,3} and whose common difference is in the set {1,2,3,4,5}?
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Solution
There are 3 ways to choose the first term and corresponding to each such way there are 5 ways of selecting the common difference. So , required no . of A.P′s=3×5=15.