label your unknown numbers I'm calling them x and y x+y=21 5x+2y=66 find a relation of x and y y=21-x substitute this into your second equation 5x+2 (21-x)=66 simplify to find x 5x+42-2x=66 3x+42=66 3x=24 x=24/3=8 apply your findings to find the second number 8+y=21 y=13 check your answer 5 (8)+2 (13)=66 40+26=66 we now know that the first number x is 8 and the second number y is 13
