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The perimeter of an equilateral triangle is 7 inches more than the perimeter of a square, and the side of the triangle is 5 inches longer than the side of the square. Find the side of the triangle. (Hint: An equilateral triangle has three sides the same length.
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From the condition, you have this system of equations:
3t = 4s + 7, (1)
t = s + 5, (2)
where t = the length of the side of the triangle, s is that of square.
To solve it, simply substitute the expression t = s + 5 of (2) into equation (1). You will get an equation for one single unknown
3*(s+5) = 4s + 7,
which is easy to solve.
s = 15 - 8 = 7.
Then t = s + 5 = 7 + 5 = 12.