What I would do:
5x + 10y = 35
Divide everything by 5.
x + 2y = 7
x = 7 - 2y
Now plug "x" into either one of the equations.
2(7-2y) + 5y = 15
14 - 4y + 5y = 15
14 + y = 15
y = 1
Now, solve for x by plugging in y into either one of the equations.
2x + 5y = 15
2x + 5 =15
2x = 10
x = 5
So x is 5 and y is 1.
Check by plugging in the values into the other equation.
5x + 10y = 35
5(5) + 10(1) = 35
25 + 10 = 35
35 = 35
The values are correct so, the answer for this problem is B (5, 1).