Three planks of different lengths, D, E and F are nailed together to make a frame as shown. Plank D has 3 holes which divide it into 4 equal parts. Plank E has 4 holes which divide it into 5 equal parts and Plank F has 5 holes which divide it into 6 equal parts. In the frame, the holes F, G and H are three corners of an equilateral triangle. Plank D is 120 cm long. What is the total length of Plank D, Plank E and Plank F?
Length of Plank D = 120 cm

Length of each interval of Plank D
= 120 ÷ 4
= 30 cm

Length of 2 intervals of Plank D
= 2 x 30
= 60 cm

Length of each interval of Plank E 
= 60 ÷ 3
= 20 cm

Length of Plank E
= 5 x 20
= 100 cm

Length of each interval of Plank F
= 60 ÷ 4
= 15 cm

Length of Plank F
= 6 x 15
= 90 cm

Total length of Plank D, Plank E and Plank F
= 120 + 100 + 90
= 310 cm

Answer(s): 310 cm