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### Question Asked by a Student from EXXAMM.com Team

** Q 2241601523. ** Let `vec(a),vec(b),vec(c)` three vectors such that `vec(a) ne 0` and

`vec(a) xx vec(b) = 2 vec(a) xx vec(c), | vec(a) | = |vec(c) | =1 , | vec(b) | =4` and `| vec(b) xx vec(c) | = sqrt (15)`, if

`vec(b) -2 vec(c) = lambda vec(a)`. Then `lambda` equals