Find S20 for the arithmetic series 5+10+15+20?

For the arithmetic series:
The first term, a = 5
The common difference, d = 10 - 5 = 15 - 10 = 20 - 15 = 5

Sₙ = n × [2a + (n - 1)d] / 2

S₂₀ = 20 × [2×5 + (20 - 1)×5] / 2
S₂₀ = 1050

S(1) = 5
S(2) = 5+10 = 15
S(3) = 5+10+15 = 30
S(4) = 5+10+15+20 = 50

In general:
S(n) = 5n(n+1) / 2

S(20) = 5(20)(21) / 2
S(20) = 1050

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S20 = (20/2) [ 10 + 19 x 5 ]
S 20 = 10 [ 105 ]
S 20 = 1050

Be careful, Jeff. "5+10+15+20" is not an arithmetic sequence; it is an expression with a value of 50. If all 20 elements have a value of 50, then S~20 = 1000.
"5, 10, 15, 20, …" is an arithmetic sequence, and Puzzling and 胡雪8° have given you correct answers.