4/7-2/5=20
20/35-14/35=6/35
42 cm3 represents 6/35 of the total volume
how many 6/35's are in 35/35 ? (the total volume)
(35/35)/(6/35)=(35/35)
*(35/6)=35/6=5 5/6
(5 5/6)*(42)=(35/6)
*(42)=35*7=245 cm3 is the total volume
so the answer is 245 cm 3 hope this helps :)