Determining SD from percentage

Hi all,

This is a question from my mid-session test, I can understand most of the others I got wrong, but I have no idea how to answer this.

*Use the following to answer questions 14 & 15*

JETSTAR's daily flight from Sydney to Broken Hill is on time seventy per cent of the time. Over the next 10 days:

Q15. The standard deviation of the relevant probability distribution (number of flights on time) is (closest to):

a) 1.45,

b) 2.10,

c) 3.00,

d) 7.00,

e) Impossible to calculate from the information given.

I'll avoid giving away my thinking to stop eliminating potential answers (because I got it wrong). Thanks folks.

P.s. This is all the information for the question.