Decision-making techniques

Could you, please, explain the following:

if sales are to be increased by 15%, labour will be paid an extra 50% on the top of the normal rate. The Revision kit gives the solution:

110 000 + ( 110 000*0.15*1.5) = 134 750

Why is 15% accounted as 0.15 instead of 1.15?

(RK p. 24, question 18.6)

Thank you in advance!