If we say it takes 56 min to fit 4 tyres to one van
Let's caunt how many min for only one tyre and one van
It will take 56 min /4 = 14 min
So it will take only 14 min for 1 tyre in one van.
Let's solve a
A) 12 tyres to 3 van.
If we will say it will take 14 min for each tyre in one van
So 12 tyres will take 14 min * 12 = 168 min which is equal 2 hours and 48 min .
B) as we said it will take only 14 min for 1 tyre.
So 42/14 =3
So he will install 3 tyres in 42 min