Question 8
The number 4 can be expressed as a sum of 1s and 2s in these five different ways:
$4=1+1+1+1$
$4=1+1+2$
$4=1+2+1$
$4=2+1+1$
$4=2+2$
In how many ways can 9 be expressed as a sum of 1s and 2s?
$4=1+1+1+1$
$4=1+1+2$
$4=1+2+1$
$4=2+1+1$
$4=2+2$
In how many ways can 9 be expressed as a sum of 1s and 2s?