Exactly eight 0's and ten 1's, if every 0 is immediately followed by a 1.
We have eight pairs of 01 and two 1's. So basically we have ten objects eight '01' and two 1's.
we have to arrange these ten objects, select 8 positions out of 10 and put '01' and in the remaining two positions put two 1's but note that two 1's can be arranged only in 1 way.
Using product rule, C(10,8)*C(2,2)= 45*1 = 45 total strings.