If 40% are boys, 60% are grills.
As fractions:
4/10 = 2/5 are boys
6/10 = 3/5 are grills
70% or 7/10 of the boys arrive by car:
(2/5)(7/10) = 14/50 = 7/25
And 60% or 3/5 of the grills arrive by car:
(3/5)(3/5) = 9/25
So in total we have 7/25 + 9/25 = 16/25 = 64/100 = 64%
64% chance that a child arrives by car.