Question 1137191
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A recursive formula tells you how to get a term of a sequence from the preceding term (or, in more complicated examples, from some combination of preceding terms).  So look for a pattern in how each number compares to the previous number:<br>
1st term 5, second term 15; to get the 2nd term, you add 10 (= 2*5) to the 1st term.<br>
2nd term 15, 3rd term 30; to get the 3rd term, you add 15 (= 3*5) to the 2nd term.<br>
3rd term 30, 4th term 50; to get the 4th term, you add 20 (= 4*5) to the 3rd term.<br>
A simple pattern should be recognizable: to get the n-th term (after the first), you add (n*5) to the (n-1)th term.<br>
In algebraic notation:<br>
t(n) = t(n-1)+5n