45machines:20mins
20mins=1/3hour
45machines:1/3hour
times 3
135machins=1hour
135machines per hour
B. 30 in 15
15=1/4 hour
30=1/4hour
times 4
120machines per hour
both produce for 2 hours
A. is 135*number of hours, B is 120 times number of hours
135*2+120*2=
270+240=
510
make 510 machines in 2 hours