F6 paye code

Hello,

I was doing an exercise from the revisions kit. It goes as follows:

Adrian is PAYE code L. Salary 15000 pounds, benefits 1160. unpaid tax from 2013/14 was 58 pounds, His personal allowance is 10600, he is a basic tax payer. what is his paye code.

The answer goes as follows

pers. all. 10600
benefits (1160)
UNPAID TAX 58×100/20

my question is, why unpaid tax is calculated in that way by multiplying 58 for 100/20?

thank you in advance