Can someone me solve these math problems? ! that would be great! u : -)

11. Sides of similar triangles are in a ratio. In this case, it is: 4:3 (big:small).

10/r = 4/3

Solving for r:

r = 7.5

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12. Plug the values you're given into both equations:
4x-7y = -1
4(5) - 7(3) = -1
20 - 21 = -1
-1 = -1
3x+5y = 30
3(5) + 5(3) = 30
15 + 15 = 30
30 = 30
Both check out, so (5,3) is a solution.
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14. 
x = 3y - 1
5y = 2x-3
You're given what x equals right away, so just plug in 3y-1 for x in the second equation. 
5y = 2(3y-1) - 3
5y = 6y - 2 - 3
-y = -5
y = 5

Then, plug y = 5 into the first equation to solve for x.
x = 3y - 1
x = 3(5) - 1
x = 14
The solution is (14,5)
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15. You need to solve for x or y in the first equation. Which one you do doesn't matter.
x+y = 5
x = 5 - y

Plug that in...
2x+5y = 19
2(5-y) + 5y = 19
10-2y + 5y = 19
10+3y = 19
3y = 9
y = 3

x = 5-y
x = 5 - 3
x = 2