Answer:
Answer C. x=10
Explanation:
Similar triangles
Given that the base lines are parallel, the base angles will be corresponding angles for the transversal and will be congruent. If one allows the up-down lines to continue to an intersection, one could form several triangles, all of which would be "Similar" to each other, due to their Angle-Angle congruence.
Similar triangles have all corresponding angles congruent between their triangles, and a consequence, all corresponding side-lengths between two triangles form a common proportion.
In other words, the ratio of the left side-length to the right side length is the same for both triangles.
Proportions
Despite that we don't actually have any full triangles, this concept extends for portions of a side length, so long as the bases are parallel.
So, since the ratio of the left side-length to the right side length is the same for both triangles, we can set up the following equation because both ratios are equal:
Solving a one-variable equation
From here, we're solving a single variable equation, where the variable only shows up once. To solve for x, we need to disconnect the "dividing by 25" from it. To undo that, we apply the opposite operation: multiplication.
Multiply both sides of the equation by 25...
Notice on the right side of the equation that x is divided by 25, and then immediately multiplied by 25. This will bring us right back to the value of x.
Computing without a calculator
On the left side of the equation, to calculate it without a calculator, we could factor each number, and cancel common factors between the numerator and denominator:
Notice that there is a 3 in the numerator and denominator. Since there is no multiplication, this is effectively starting with 2, multiplying by 3, and then immediately dividing by 3, which will just bring us back to 2, before dividing by 5. So, the 3s cancel, as collectively, they won't change the value of 2.
Now, we have the same process with the 5s. 2 is first divided by 5, and then immediately multiplied by 5 (before being multiplied by another 5). The division by 5 and the first multiplication by 5 will cancel, as they collectively won't change the value of the 2.
Lastly, 2*5 is 10
If you were allowed to use a calculator to calculate it, then that simplifies the work to getting that answer.