a) 186 minutes
b) 3 tyres
We can take 124 and divide it by 4 to see how long it takes him to fit 1 tyre, then use that number to sort the rest out.
124/4 = 31
It takes Liam 31 minutes to fit one tyre to a lorry.
To fit 6 tyres, it takes 6 x 31 = 186 minutes
If he works for 93 minutes,
93/31 = 3 tyres
Please let me know if you have any questions.