Pagalpan of Professor

Professor to student: 1 platform 2km lamba hai,
aandhi chal rhi hai, 60 km/hr. ki speed se ek train aayi aur agra se delhi ki taraf chaligai..
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To sawal ye hai k, "batao meri umar kitni hai.........?? . .
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Ek student kafi der baad bola: 42 saal hai. .
Professor: Good, lekin tumko kaise calculate kia......?? .
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Student: Sir, hamare ghar k paas ek admi rehta hai woh adha pagal hai aur uski umr 21 saal hai..!!