Let x = the 1st number, and y = the 2nd number.
Equation 1: The second of two numbers is 4 times the first ==> y = 4x
Equation 2: Their sum is 50 ==> x + y = 50
Substitute y = 4x into Equation 2, you get
x + 4x = 50
5x = 50
x = 50 / 5 = 10
y = 4x = 4(10) = 40
