We can assume that 5x+4 = 10x-3 if they are twins and the same age, so take 5x from both sides to get 4=5x-3, add 3 to get 7=5x, and divide by 5, so x=1.4. We can then sub this value for x back into one of the equations, so (5x1.4)+4 to see that they are both 11 years old.
