Hello, xiaoz!

Seven men can paint a bridge in 15 days.

(1) How long woud it take 3 men?

One man can paint of the bridge in 15 days.

In one day, one man can paint of the bridge.

Hence, 3 men can paint of the bridge in one day.

Therefore, it will take the 3 men days to paint the bridge.

(2) The bridge was painted in days.

Write an expression, in terms of ,

for the number of men needed to paint the bridge.

Some information is missing . . .