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The summation of an arithmetic series is probably the best way to approach this problem.
So he makes 24 calls the first day, 29 the next, etc. for 20 days total.
How many calls does he make on the last day? 24 + 5(19) = 24 + 95 = 119
So by the formula, he made $\displaystyle \frac{20(24 + 119)}{2}$ total calls, which is 1430.