When the supply voltage to an induction motor is reduced by 10%, the maximum torque will be decreased approximately by
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For small percentage changes, you can use calculus approximations (binomial theorem):
$$T \propto V^2 \quad \Rightarrow \quad \frac{\Delta T}{T} \approx 2 \cdot \frac{\Delta V}{V}$$
Given a $10%$ reduction in voltage ($\frac{\Delta V}{V} = 10%$):
$$\frac{\Delta T}{T} \approx 2 \times 10% = 20%$$
This approximation provides the correct answer immediately.