hard · GMAT Quant
A traffic controller sets a 4-phase signal timing using modes Red (R), Yellow (Y), and Green (G). Rules mandate that phase 1 cannot be Yellow (Y), phase 4 cannot be Red (R), and no mode can be selected in two consecutive phases.
How many valid 4-phase timing sequences can be generated?
- 8
- 11
- 16
- 24
- 54
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