Hi, 

Four times a number minus 5 times another number is equal to 21.

Inserting x and y in the sentence

Four times a number  minus 5 times another number  is equal to 21

Gives us:  4x - 5y = 21

Three times the sum of the two numbers is 36

Gives us:  3(x+y) = 36   0r  x + y = 12

Two Equations, Two Unknowns

 4x - 5y = 21

  x +  y = 12  

Multiplying 2nd Equation by 5 to eliminate y

 4x - 5y = 21

5x + 5y = 60

 9x = 81

   x = 9   and y = 3   

Two Numbers are 9  an 3

checking our Answer

 4x - 5y = 21

 36 - 15 = 21  checks.

Wish You the Best in your Studies.

 

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