Question 938939
Pipe A fills {{{1/8}}} pool per hour.{{{1/8}}}*100%=12.5% per hour
Pipe B fills {{{1/12}}} pool per hour.{{{1'12}}}*100%=8.3% per hour
Convert the 90 minute fill time to hours: 
{{{90 minutes/(60 minutes/hr)}}}=1.5 hour
Pipe A fills 12.5% of pool/hr x 1.5 hours=18.75% of pool
Together Pipes A and B fill (12.5%+8.3%)=20.8% of the pool per hour.
After 1.5 hours of just Pipe A, 100%-18.75%=81.25% is left to fill.
81.25% of the pool/20.8% per hour (both Pipes)=3.9 hours to finish filling.
1.5 hours(just Pipe A) + 3.9 hours(both Pipes)=5.4 hours total to fill pool.