Moderate -0.8 This is a straightforward application of standard arithmetic progression formulas requiring only three steps: find the common difference from a + 19d = 72, then apply S_n = n/2(2a + (n-1)d) with n=100. It's more routine than average since it involves direct formula substitution with no problem-solving insight needed, though the arithmetic with larger numbers prevents it from being trivial.
In an arithmetic progression the first term is 15 and the twentieth term is 72. Find the sum of the first 100 terms. [4]
\hfill \mbox{\textit{OCR C2 2007 Q1 [4]}}