Before selling, Farmer Tan had 100u

After selling 45%, he had 55u left.

55u = 495

45u = 495 ÷ 55 × 45 = 405

Farmer Tan sold 405 chickens, and Farmer Lam bought 20% of them.

20% × 405 = 81

495 – 81 = 414

Farmer Lam had 414 chickens at first

Thanks you very much for the solution