Because every driver who paid £1.5 with a £1 coin must have also paid with a single 50p coin, you know that that 136 of the 50p coins must have been paid by the drivers who did not pay three 50p coins. So 208-136 = 72.

So 72 of the 50p coins must have been paid by drivers who paid three 50p coins.

Since each driver who paid with 50p coins paid three, then you know that there are one third as many drivers as coins, so 72/3 = 24

So we have the total number of drivers being 136+24 = 160

and the percentage of drivers who paid with 50p coins is 24 divided by 160 = .15

So 15% of the drivers paid with three 50p coins.