Acarpenter cuts of from a plank 5/12 of its length and then 6/7 of what remains. of the remaining piece is 2/1/2 m long. find the original length of the plank

Cut 5/12 of his lenght

remain 7/12 of his lenght
 
7/12*L - 6/7 *7/12*L=2 1/2 m
7/12*L(1-6/7)=7/12*L(7/7-6/7)=7/12*L*1/7=L/12 =2 1/2

⇒L=2 1/2 m *12=5/2*12=30 m

Original length was 30 m