Make use of the fact that side lengths in similar triangles are proportional.
.. 30/18 = (20 +x)/20
.. 20*(30/18) -20 = x . . . . multiply by 20, subtract 20
.. 33 1/3 -20 = x . . . . . . . . evaluate the product
.. 13 1/3 = x
Try to get all the x'es to the right side
The answer is .5. That is the answer because .5/10 = .05.
5384=5000+384
4 * 5384 = 4(5000+384) = 4 * 5000 + 4 * 384
4 * 5000 = 20000
20000 + 4 * 384
So to find 5384 * 4, you add 20000 to 4*384. Less calculations