Dany, this is the solution:
The error occured at the first step:
We should add 0.05 and 0.1 and not subtract them.
Therefore, the expression should be:
3x + x + x + 0.15 = 7.50
5x + 0.15 = 7.50
Subtracting 0.15 at both sides:
5x + 0.15 - 0.15 = 7.50 - 0.15
5x = 7.35
Dividing by 5 at both sides:
5x/5 = 7.35/5
x = 1.47