First we put ‘+’ in a line.
This can be done in 1 way because all ‘+’ are same.
+ + + + + +
Now we put ‘-‘ in the following way
– + – + – + – + – + – + –
Hence, the method of putting ‘-‘ sign in 4 out of 7 places = 7C1 × 1 because ‘-‘ signs are same.
Total number of ways 6 ‘+’ and 4 ‘-‘ sign occur together