Answer:
? = 4.29
Explanation:
Remark
Let x be the question mark. You get the proportionality by using the dimensions of the little triangle to the dimensions of the large triangle.
Equation
14/(14+ 4) = 15/(x + 15) Combine like terms on the left
14/18 = 15/(x + 15) Cross multiply
14*(x + 15) = 18 * 15 Simplify the right
14(x + 15) = 270 Divide by 14
x + 15 = 270 / 14
x + 15 = 19.29 Subtract 15 from both sides
x = 19.29 - 15
x = 4.29