Question

A student has a mean score of 88 on five tests taken. What score must she obtain on her next test to have a mean (average) score of 80 on all six tests?
Mean score of tests :

( x1 + x2 + x3 + x4 + x5 ) / 5 = 88 Multiply both sides by 5

x1 + x2 + x3 + x4 + x5 = 88 * 5

x1 + x2 + x3 + x4 + x5 = 440


Mean score of six tests :

( x1 + x2 + x3 + 4 + x5 + x6 ) / 6 = 80

[ ( x1 + x2 + x3 + 4 + x5 ) + x6 ] / 6 = 80

( 440 + x6 ) / 6 = 80 Multiply both sides by 6

440 + x6 = 80 * 6

440 + x6 =480

x6 = 480 - 440

x6 = 60

( x1 + x2 + x3 + x4 + x5 + x6 ) / 6 = 80

x6 = 480 - 440

x6 = 40

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