What is the x in 2/5 + 4/{10x+}) = 7/{2x+1}?

(2/5) + 4/(10x + 5) = 7/(2x + 1)  

Note that 

4/(10x + 5)   = 4/[5(2x + 1)] = (4/5)/(2x + 1)

So we have

(2/5) + (4/5)/(2x + 1) = 7(2x + 1)     mutiply through by 5 to get rid of the fractions

2 + 4/(2x + 1) = 35/(2x + 1)              subtract 4/(2x + 1) from both sides

2 = 35/(2x + 1) - 4/(2x + 1)               simplify

2 = 31/(2x + 1)                                  cross-multiply

2(2x + 1) = 31                                    simplify

4x + 2  = 31                                       subtract 2 from both sides

4x = 29                                              divide by 4 on both sides

x = 29/4