How about trying to make 90 using the digits 1-8 and leaving the 9
for adding on at the end?<br />

A good place to start may be to push the 1 and 2 together to make
12. What will you still need to add onto 12 to make 90? Can you
make 90 simply by adding the remaining digits together? What could
you do?<br />

Perhaps you could try pushing the 2 and 3 together to make 23? What
do you need this time to make 90?