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First you draw 2 equal models to represent the number of identical flight of stairs. Note: this means that there are equal number of steps in each flight of stairs. For the top model, draw 4 run steps (shown in red) and name the remainding as steps that were walked (in pink and yellow). The bottom model, draw 11 steps (shown in red) and name the remainding as steps that were walked (in pink).

Since running steps will take the same time, you can cancel the 4 red units at the top with the bottom 4 red unit. Similiarly, since the walking steps take the same time, you can cancel the yellow units on top with the bottom one. The yellow unit represents a certain unknown number but they have to be the same number.

Thus when you take 75 - 40 = 35

This leaves you with the difference caused by running the 7 steps in red versus the walking steps in pink. In other words, walking the 7 steps take 35 seconds more than running the 7 steps. Note: the pink units also represents 7 steps as you have begin with the same number of stairs

35/7=5 Walking each step takes 5 sec more than running each step.

To find out how long he take to finish walking one flight of steps, change the 4 running steps to walking by adding a 5 sec each. Then add it to the 75 sec.

5x4= 20

20+75=95

2 flights of stairs = 95x2=190sec

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