To find $30\%$ of $1.5$ liters, multiply by $0.30$: $1.5\times 0.30=0.45$ liters. Then convert liters to milliliters by multiplying by $1000$: $0.45\times 1000=450$ milliliters. So the correct expression is $1.5\times 0.30\times 1000$.

From the points on the graph, the faucet fills $10$ gallons in $2$ minutes, so the rate is $\frac{10}{2}=5$ gallons per minute. After $8$ minutes, the tank will contain $8\cdot 5=40$ gallons. Since the tank holds $50$ gallons total, the fraction filled is $\frac{40}{50}=0.8$, which is $80\%$. Therefore, the correct answer is $80\%$.

The total amount of mixture is 30 ounces, so the first equation is $j+w=30$. Since 20% of the mixture is juice, the amount of juice is $0.2\times 30=6$, so the second equation is $j=0.2(30)$. Therefore, the correct system is $j+w=30$ and $j=0.2(30)$.

First find the total number of visitors: $240+300+210=750$. Then find $25\%$ of 750. Since $25\%=0.25$, compute $0.25\times 750=187.5$. So the number of student visitors was $187.5$.

To find how much water remains, first calculate 25% of 80 liters: $0.25(80)=20$. That means 20 liters were used. Subtract from the original amount: $80-20=60$ liters remain. Since 60 is greater than 55, the student's claim must be true.