exam Maths Puzzle Numerical Ability

Does anyone understand this question?

The standard deviation of a distribution is 20. If a sample of 225 is selected, what is the standard error of the mean?

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exam Other Question

Solve the following system by using either addition or substution. If a unique solution does not exist, state whethher the system is depent or inconsistent.

10x + 2y = 7
y = -5x + 3 Sales--Tickets for a concert were sold to adults for $3 and to students for $2.
If the total sold were the amount of $ 824 and twice as many adult tickets were
sold,then how many of each were sold?