If 1st July, 1977 was a Friday then 1st July 1970 was a

Every year has 365 days, which increases the weekday by one. During leap years, the week day is increased by 2. So, adding up we get

70 - 71 => 1 day increase
71 - 72 => 2 day increase
72 - 73=> 1 day increase
73 - 74=> 1 day increase
74 - 75=> 1 day increase
75 - 76=> 2 day increase
76 - 77 => 1 day increase

From 70 to 77, there is an increase of 9 weekdays. ie 2 weekdays ( 9 = 7 + 2 )
So, if 1st July, 1977 was a Friday then 1st July 1970 was a Wednesday