#5/a=30/54# => simplify: #30/54=5/9# then: #5/a=5/9#=>equal fractions with equal numerators, then the denominators must be equal so: #a=9#=> let's prove it: #5/a=5/9#=>simplify further divide both sides by 5: #1/a=1/9#=>cross multiply: #1*a=1*9#=>or: #a=9#