Mathematics

An arithmetic progression is defined by a starting point and a constant step size—here you know both the common difference and one specific term. The key insight is to use the general term formula $a_n = a_1 + (n-1)d$ to first find the first term, then use it again to locate where 85 appears. You're essentially working backwards from a known term to find the starting point, then forwards to your target.

An arithmetic progression is defined by a starting point and a constant step size—here you know both the common difference and one specific term. The key insight is to use the general term formula $a_n = a_1 + (n-1)d$ to first find the first term, then use it again to locate where 85 appears. You're essentially working backwards from a known term to find the starting point, then forwards to your target.