Step 1: Assume Z's income

Let the income of Z be $100$.

$Z = 100$

Step 2: Find Y's income in terms of Z

Since the income of Y is $40\%$ more than Z's income: $Y = 100 + 40\% \text{ of } 100 = 140$

$Y = 140$

Step 3: Find X's income in terms of Y

Since the income of X is $60\%$ more than Y's income: $X = 140 + 60\% \text{ of } 140 = 140 + \frac{60}{100} \times 140 = 140 + 84 = 224$

$X = 224$

Step 4: Calculate percentage by which X's income is more than Z's income

Difference between X and Z is $224 - 100 = 124$. Expressed as a percentage of Z: $\text{Percentage more} = \frac{224 - 100}{100} \times 100\% = 124\%$

$124\%$