a). integer between 50 and 100 and all no divisible by 7=( take integer between 1 and 100 and find all no divisible by 7)minus from(take ineger between 1 and 49 and find all no divisibility by 7).
now first integer will be 7 and last will be 98 between 1 to 100 who is divisible by 7.we can use A.P. concept for finding no of element Tn=a+(n-1)d whose first term (a)is 7 ,last term ( Tn)is 98 and common difference(d) is 7.
now,98=7+(n-1)7 we get n=14
similarly between 1 and 49 ,first term =7,last term=49,common difference d is 7 find all no using Tn=a+(n-1)d .49=7+(n-1)7,we get n=7 from here,
now final result=14-7 =7
we can use similar concept for b and c as well.