This is a classic speed-distance-time problem, and once you set it up correctly, the math is very clean.
Step 1: Define variables.
Let John's usual speed be s, and let the usual time to reach work be t. Then the distance to work is d = s * t.
Step 2: Set up the equation for today.
Today he walks at 3/5 of his usual speed, so his speed is (3/5)s. Since the distance doesn't change:
d = (3/5)s * t_new
Since d = s * t, we get:
s * t = (3/5)s * t_new
t_new = (5/3) * t
Step 3: Use the "10 minutes late" information.
The extra time is:
t_new - t = (5/3)t - t = (2/3)t = 10 minutes
Solving: t = 10 * (3/2) = 15 minutes.
Answer: B (15 minutes)
Common trap: Some students set up the ratio backwards and get 25 minutes. Remember — slower speed means more time, so t_new must be greater than t. The multiplier for time is the inverse of the speed fraction: speed is 3/5 of normal, so time is 5/3 of normal.
Takeaway: Whenever a speed-distance-time problem gives you a fraction of the usual speed and the resulting time difference, set up the equation (new time) - (old time) = difference. The distance cancels out, and you're left with a simple equation.