Heather has sewn 13 x 3 = <<13*3=39>>39 aprons today.
The total number of aprons sewn is 13 + 39 = <<13+39=52>>52.
Heather needs to sew 150 - 52 = <<150-52=98>>98 aprons more.
So, she needs to sew 98/2 = <<98/2=49>>49 aprons tomorrow.
#### 49