After five tests, mike's average score is 91. after 6 tests, his average score was 89. what was his score, on the sixth test?

( x 1 + x 2 + x 3 + x 4 + x 5 ) / 5 = 89 Multiply both sides by 5 

x 1 + x 2 + x 3 + x 4 + x 5 = 89 * 5 = 445 

( x 1 + x 2 + x 3 + x 4 + x 5 + x 6 ) / 6 = 90 

( 445 + x 6 ) / 6 = 90 Multiply both sides by 6 

445 + x 6 = 6 * 90 = 540 

x 6 = 540 - 445 = 95

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