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1. Can someone explain the last two parts of the question for me, please?

'The diagrams show keypads for two different types of alarm.
Each keypad has a four-key code.
(a) How many different codes are possible for the
(i) Classic alarm pad = 10^4= 10000
(b) The Premier alarm keypad is then programmed so that the four-key code must start withh two letters, followed by two digits. Show tht there are fewer than 1000 codes possible.
The answer to (a) (ii) is 28561
and (b) is 'There are 900 possible codes'
2. (Original post by BT144)
Can someone explain the last two parts of the question for me, please?

'The diagrams show keypads for two different types of alarm.
Each keypad has a four-key code.
(a) How many different codes are possible for the
(i) Classic alarm pad = 10^4= 10000
(b) The Premier alarm keypad is then programmed so that the four-key code must start withh two letters, followed by two digits. Show tht there are fewer than 1000 codes possible.
The answer to (a) (ii) is 28561
and (b) is 'There are 900 possible codes'
For your second problem, there are 9 ways of choosing 2 letters from 3 where ordering is unique. This is 3^2, but you can see from ABC that it can be AA, AB, AC, BA, ... CC. (Total = 9)

So 9 ways of choosing the first two of the 4 characters. So now you need to choose two digits and combine the two ideas.

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Updated: February 11, 2018
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