question on NIC

Bob is an employee of Dibbit Ltd and is paid an annual salary of £50000.He makes contributions of £3000 into the company’s occupational pension scheme each year.

In the tax year 2014/2015 he drove 12000 business miles in his own car for which he was paid 50 p per mile by Dibbit ltd

How much employer’s class 1 NICs is Dibbit Ltd required to pay in respect of Bob’s earnings and refunded mileage for the tax year 2015/16?
answer
class 1 secondary NIC £
(50600-£8112)*13.8%=£5863
w:EARNINGS liable to class 1 NIC
SALARY 50000
EXCESS MILEAGE ALLOWANCE 50000*(50p-45p)=600
50600

we didnt need to deduct contributions from salary and for business miles wasnt it necessary for first 10000miles to be charge 45p
and above 10000miles 25p