SOLUTION: I am a number between 70 and 80. The second digit is 3 smaller then the first digit. I know it's 74 because 7-4=3, but how can i solve it mathematically?

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Question 1036895: I am a number between 70 and 80. The second digit is 3 smaller then the first digit. I know it's 74 because 7-4=3, but how can i solve it mathematically?
Found 2 solutions by Boreal, MathTherapy:
Answer by Boreal(15235)   (Show Source): You can put this solution on YOUR website!
70<=10y+x<=79
y-x=3
====
70<70x-210+x<=79
280<=71x<=289
x is an integer
x=4; y=7
Answer by MathTherapy(10810)   (Show Source): You can put this solution on YOUR website!

I am a number between 70 and 80. The second digit is 3 smaller then the first digit. I know it's 74 because 7-4=3, but how can i solve it mathematically?
Let the tens and units digits be T and U, respectively

Then the number is: 10T + U 

Since it's between 70 and 80, T MUST be 7, and so the number is: 10(7) + U, or 70 + U

Also, it's given that the second (units) digit is 3 smaller than the first (tens) digit, so we can say that: U = T - 3, or U = 7 - 3, or 4

With the tens digit being 7 and the units digit, 4, the number's:  

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