\begin{align*} \require{cancel} \text{Let length of cube} & = x \\ \\ \text{Original volume} & = x \times x \times x \\ & = x^3 \\ \\ \text{New length of cube} & = x \times {110 \over 100} \phantom{00000} [100\% + 10\% = 110\%] \\ & = x \times 1.1 \\ & = 1.1x \\ \\ \text{New volume} & = 1.1x \times 1.1x \times 1.1x \\ & = 1.331 x^3 \\ \\ \text{Percentage change} & = { \text{Final} - \text{Initial} \over \text{Initial} } \times 100 \\ & = { 1.331 x^3 - x^3 \over x^3} \times 100 \\ & = { 0.331 \cancel{x^3} \over \cancel{x^3} } \times 100 \\ & = 0.331 \times 100 \\ & = 33.1 \% \end{align*}

\begin{align*} \require{cancel} \text{Let length of rectangle} & = x \\ \\ \text{Let breadth of rectangle} & = y \\ \\ \text{Area of rectangle} & = x \times y \\ & = xy \\ \\ \text{New length} & = x \times {115 \over 100} \\ & = x \times 1.15 \\ & = 1.15x \\ \\ \text{New breadth} & = y \times {80 \over 100} \\ & = y \times 0.8 \\ & = 0.8y \\ \\ \text{New area} & = 1.15x \times 0.8y \\ & = 0.92xy \\ \\ \text{Percentage change} & = { \text{Final} - \text{Initial} \over \text{Initial} } \times 100 \\ & = { 0.92xy - xy \over xy} \times 100 \\ & = { -0.08 \cancel{xy} \over \cancel{xy} } \times 100 \\ & = -0.08 \times 100 \\ & = -8 \% \end{align*}

\begin{align*} \text{Timing in 2026} & = 9 \text{ minutes 54 seconds} \\ & = (9 \times 60) + 54 \\ & = 594 \text{ seconds} \\ \\ [2025 \rightarrow 100\%, & \phantom{.} 2026 \rightarrow 90\%] \\ \\ \text{Timing in 2025} & = 594 \times {100 \over 90} \\ & = 660 \text{ s} \\ \\ [2024 \rightarrow 100\%, & \phantom{.} 2025 \rightarrow 66\%] \\ \\ \text{Timing in 2024} & = 660 \times {100 \over 66} \\ & = 1000 \text{ seconds} \\ \\ 1000 \text{ seconds} & = {1000 \over 60} \\ & = 16{2 \over 3} \text{ minutes} \\ & = 16 \text{ minutes } {2 \over 3} \times 60 \text{ seconds} \\ & = 16 \text{ minutes 40 seconds} \end{align*}

\begin{align*} \text{Cost price} & = \$180 \\ \\ \text{Selling price after discount} & = 180 \times {120 \over 100} \phantom{000000} [20 \% \text{ profit}] \\ & = \$216 \\ \\ \text{Marked price} &\rightarrow 100\% \\ \text{Selling price after }10\%\text{ discount} &\rightarrow 90\% \\ \\ \text{Marked price} & = 216 \times {100 \over 90} \\ & = \$240 \end{align*}

\begin{align*} \text{Fraction of pastries that were croissants} & = 40\% \\ & = {40 \over 100} \\ & = {2 \over 5} \\ \\ \text{Fraction of pastries that were curry puffs} & = \left(1 - {2 \over 5}\right) \times 60\% \\ & = {3 \over 5} \times {60 \over 100} \\ & = {9 \over 25} \\ \\ \text{Fraction of pastries that were egg tarts} & = 1 - {2 \over 5} - {9 \over 25} \\ & = {6 \over 25} \\ \\ \text{Total number of pastries} & = 36 \times {25 \over 6} \\ & = 150 \end{align*}