If there are seven steps, it is necessary to reach the 7th step, but all other previous steps are optional.
Let's assume that the binary sequence:
0 0 0 1 0 1 1
represents a mode using steps 4, 6 and 7.
To find the total number of ways to climb seven steps, we need to find out how many binary sequences there are using 6 binary digits (since step 7 will always be 1).
We have then 26 = 64 ways to climb the seven steps.
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