Three planks of different lengths, B, C and D are nailed together to make a frame as shown. Plank B has 3 holes which divide it into 4 equal parts. Plank C has 4 holes which divide it into 5 equal parts and Plank D has 5 holes which divide it into 6 equal parts. In the frame, the holes S, T and U are three corners of an equilateral triangle. Plank B is 312 cm long. What is the total length of Plank B, Plank C and Plank D?
Length of Plank B = 312 cm

Length of each interval of Plank B
= 312 ÷ 4
= 78 cm

Length of 2 intervals of Plank B
= 2 x 78
= 156 cm

Length of each interval of Plank C 
= 156 ÷ 3
= 52 cm

Length of Plank C
= 5 x 52
= 260 cm

Length of each interval of Plank D
= 156 ÷ 4
= 39 cm

Length of Plank D
= 6 x 39
= 234 cm

Total length of Plank B, Plank C and Plank D
= 312 + 260 + 234
= 806 cm

Answer(s): 806 cm