What do you know? You're exactly the same height as your 5-foot longbow, and your shadow
is exactly the same length as 2.5 longbows. You
get an idea and measure the length of the
tower's shadow: 20 longbows. Yes, you've got
enough information to figure out how tall the
tower is now!

So the first thing we do is to multiply the top equation by 3. the reason i want to do this is i notice there will be a 6y on both equations and can possibly add/subtract them. so we get 6y = -3x + 27. adding that and the second equation we get 6x = 27-15 which is 12. divide by 6 and we get 2. ∴ x = 2. substituting 2 into the first equation we get 2y = -2 + 9 which is 7. divide by 2 and we get 7/2. ∴ y = 7/2, x = 2.