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The sum of -4 and 8 times a number is multiplied by 3. The result is 15 less than 25 times the number. What is the number?

Let the number be x

the first sentence results in 3(-4 + 8x)

the second sentence is (25x - 15)

and these two are equal

3(-4 + 8x) = 25x - 15

-12 + 24x = 25x - 15 ; add 15-24x to both sides

3 = x