How many arithmetic progressions with 10 terms are there, whose first term is in the set {1,2,3,4} and whose common difference is in the set {3,4,5,6,7} ?
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Solution
The first term can be any of the three numbers from the set 1,2,3&4. Therefore, the first term can be chosen in 4 ways. The common difference
can be chosen in 5 ways. Hence total number of ways=4×5=20