Answer- b+t=90 (or bb + tt = 90 ignore if this was a mistake)
If we know that 90 is an absolute total, that will go behind the equal sign
?=90;
We know that we can make any mixture of burritos or tacos, but they have to equal up to ninety. As you stated bb= burritos and tt= tacos. (If that is a mistake, instead of putting bb or tt, just put the single letter b or t)
This means our equation would have to have a certain number times burritos and a certain number times tacos that equal 90.
Our equation should look a little like this
(number of burritos ) + (number of tacos) = 90
b + t =90 or with your values bb+tt=90
HOPE THIS HELPS & GOOD LUCK