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| challenge (MichaelBurov); brain teaser (син. brain-twister; puzzle, problem, riddle, conundrum, poser Taras) | |||
| weighty problem | |||
| difficult nut to crack (Violetta Gun.); insidious problem (Wolverin) | |||
| intractable problem | |||
| formidable task (ssn) | |||
| трудноразрешимая задача: 1 phrase in 1 subject |
| Mathematics | 1 |