The true value of angle of dip at a place is 60o. The apparent dip in a plane inclined at an angle of 30o with magnetic meridian is

tan ϕ'=tan ϕcos β; where ϕ'=Apparent angle of dip, ϕ = True angle of dip, β =Angle made by vertical plane with magnetic meridian.⇒tan ϕ'=tan 60ocos 30o=2⇒ϕ'=tan-1(2)