Question 570535
Let's say that the first three numbers are: a, b, and c. We need to find their sum.
{{{(a+b+c)/3 = 42}}} "The average of the first three numbers was 42".
So the sum of the first three numbers is {{{3(42) = 126}}}
The next seven numbers we'll identify as: d, e, f, g, h, i, j and their average is:
{{{(d+e+f+g+h+i+j)/7 = 12}}} "The average of the next seven numbers was only 12". So their sum is {{{7(12) = 84}}}
So we have 10 numbers altogether.  To find their average, divide their sum by the number of numbers (that's 10):
{{{(126+84)/10 = 210/10}}}={{{21}}}
The average of all ten numbers is 21.