Hey there!
"The second of two numbers..." means that whatever follows will be referring to the second number in this problem.
"...is four times the first..." means that the second number will be 4x if the first number is represented by x.
"...Their sum is fifty." means that the entire equation will be set to equal 50 and the equation itself is an addition problem, since sum always equates to addition.
So, the final equation will be x + 4x = 50. To solve this, all we need to do is simplify.
x + 4x = 50
1x + 4x = 50
5x = 50
5 5
x = 10
So, the first number (x) will be 10 and the second number (4x) will be 4 times x, (or 4 times 10) or 40. You could check this by simply adding up these numbers:
10 + 40 = 50
Hope this helped you out! :-)