Answer:
86%
Step-by-step explanation:
Let g represent the mean mark for the 10 girls in the class. Then the mean mark for the class was ...
10g +20(62%) = 30(70%)
g = 3(.7) -2(.62) . . . . . . . . . divide by 10 and subtract 2(.62)
g = .86 = 86%
The mean mark for girls was 86%.
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You can do this in your head if you consider it in terms of deviations from the mean. The second equation above can be rewritten and factored as ...
g = 70% +2(70% -62%)
That is, because there are 2 boys for every girl, the girl's score is above the mean by an amount that is 2 times the amount that the boy's score is below the mean. The boys' score is below by 8%, so the girls' score is above by 16%, so is 86%.
Shortest is 3/4 longest is 1 1/2. 1 1/2 -3/4=3/4
4(x+7) becomes 4x+28 you multipli the 4 in parentheses ans 2(x+7) becomes 2x+14
4x+28=2x+14 you move the x termes to the left and other numbers to the right look:
4x-2x=14-28 when you change the sides always change the sign +or-
2x=-14
2x/2=-14/2
X=-7
20 seconds faster than Jesse so one minute and 57 seconds is my best guess.
