If the answer is 90

the questions could be

1.50+40=90
2.80+10=90
3.89+1=90
4.20+70=90
5.40+50=90
6.10+80=90
7.101-11=90
8.10x9=90
9.81+9=90
10.60+30=90
11.86+4=90
12.95-5=90
13.100-10=90
14.87+3=90
15.90+0=90
16.30+60=90
17.70+20=90
18.105-15=90
19.110-20=90
20.102-12=90