What is the average (arithmetic mean) of integers j, k, and l?

(1) The sum of j and k is 9.
(2) The sum of j and l is 10.

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I thought the answer was C, but it is E. Can someone explain please? Thanks!

Arithematic mean = (J+K+L)/3

statement (1) J+K = 9. No information about L. INSUFFICIENT
statement (2) J+L = 10, No information about K. INSUFFICIENT

Combine both

2J+K+L = 19 still cannot calculate J+K+L, Insufficient