My son asked me this question. The answer to this question is 45. But I don't know how to get to that answer.
Question:
John can get to school in 12 minutes riding his bike at an average of 15 miles per hour (mph). How many minutes would it take walk to school if he walks at 4 mph?
It's always good to read the question carefully when you solve these problems. The statements suggests that a person can ride his bike $12$ mph for 15 minuts (asuming that his speed is constant all the way). The next statement tells us that he can walk $4$ mph to school. Based on these statements, an equation can be formed very easily. First off, it is quite obvious that the units are not the same - we need to convert them. Therefore, $\frac{15}{60}(12)=\frac{4}{60}(x)$ Here is the explaination:
The left side represents the total distance of his walk to school. Notice that rate times time equals distance? However, the units are not the same in this case. This is why $\frac{15}{60}$ is required. $\frac{15}{60}$ just means that 15 mile per 60 minutes, ($\frac{1}{4}$ miles per minute if you simplified this). The right side simply represents the distance also. However, we do not know the time. Again, we need to do the unit conversion to solve the problem properly. Now, since an equation is provided, we can solve it! $\frac{1}{4}(12)=\frac{1}{15}(x)$ Next, multiply the left side by 12. $\frac{12}{3}=\frac{x}{15}$ $3=\frac{x}{15}$ Now, multiply both sides by $15$ to cancel the fraction. $x=15*3$ $x=45$ So, it will take him 45 minutes.