Let [tex]c[/tex] denote the number of customers.
The number of customers that seek advice is [tex]0.3c[/tex].
The number of customers that do not seek advice is thusly [tex](1-0.3)c=0.7c[/tex]
40% of the customers that do seek advice buy a device, denoted with [tex]0.4\cdot0.3c=0.12c[/tex] that is 12% of the total customers here buy a device.
20% of the customers that do not seek advice buy a device, denoted with [tex]0.2\cdot0.7c=0.14c[/tex] so 14% of the total customers here buy a device.
Together, the percentage of customers that will buy an applicance on the end of the day is [tex]12\%+14\%=26\%[/tex].
So the probability from that follows that when a customer enters the shop there exists a 26% probability that the customer will end up buying a device.
Hope this helps :)