Answer this word problem (statistics)

Question

The mean of a set of test scores is 64. A new student takes the test and scores 80. When his score is added to the other scores, the mean increases to 65. How many students sat for the test?

Answer 1

AVERAEJ=64, so oldsum=64n

1 nu persun...nusum=oldsum+80 & nu averaej=65

64n+80=65*(n+1)

64n -65n +80=65

-n=65-80=-15

n=15

oldsum=15*64=960

nusum=960+80=1040

1040/16=65