exam Maths Puzzle Numerical Ability

A student wrote 69 as the average of the data: 70,59,72,35,67,78,68,81,80,and 76. By how much was the answer incorrect?

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exam Other Question

Jack gave 1/2 of her apple to Jill, Jill gave 1/3 of the apple he received to Pete and Pete gave 1/4 of what he received to Mike, if Mike received 24 apples, how many did Jack heve originally? I need help with the Echelon Method on this problem
x-y+z=4
x+y+z=10
2x+y-3z=7
I understand the gauss jordan method to solve this but not the echelon?
3 variables throws me o