If the coding stage was performed in the $5^{th}$ month, then the cost will be $10,000 $ per man-month which is $10,000 \times 4 = 40,000$ because the coding stage was supposed to perform by $4$ people.
But,
now the design stage is performed in the $5^{th}$ month & $5$ people are working on that stage and for performing or doing the design stage the cost is $20,000$ per man-month, & the total cost will be $20,000 \times 5 = 1,00,000$ in the $5^{th}$ month.
In the $5^{th}$ month, the cost was supposed to be $40,000$
& now the cost incurred = $1,00,000$
Percentage change in the cost incurred in the $5^{th}$ month = $\dfrac{(1,00,000-40,000)}{40,000} \times 100\% \\ = \dfrac{60,000}{40,000} \times 100\% \\ = \dfrac{6}{4} \times 100\% \\ = \dfrac{3}{2} \times 100\% \\ = 150\%$