i udn understand question.. help mi wif tis questions
Hello, xiaoz!
Seven men can paint a bridge in 15 days.
(1) How long woud it take 3 men?
One man can paint $\displaystyle \frac{1}{7}$ of the bridge in 15 days.
In one day, one man can paint $\displaystyle \frac{1}{105}$ of the bridge.
Hence, 3 men can paint $\displaystyle 3 \times \frac{1}{105} = \frac{1}{35}$ of the bridge in one day.
Therefore, it will take the 3 men $\displaystyle 35$ days to paint the bridge.
(2) The bridge was painted in $\displaystyle t$ days.
Write an expression, in terms of $\displaystyle t$,
for the number of men needed to paint the bridge.
Some information is missing . . .
Greetings Xiaoz:
If y varies directly with the square of x, then the relation between x and y is: y = kx^2, where k is a constant. Given y = 10 for some value of x, which I shall call xt, it follows that 10 = k(xt)^2. Now, dividing xt by 2 gives k(xt / 2)^2 on the right. And elimination of parentheses leaves k*(xt)^2 / 4. Hence dividing xt by 2 has the effect of dividing k(xt)^2 by 4. (This is only logical, yes? Dividing a squared quantity by 2 is equivalent to inserting factor, 1/2, within the 'squared parentheses'. The action yields 1/4 upon squaring). Anyway, if k(xt)^2 = 10, then it follows that [k(xt)^2] / 4 = 10/4 = 5/2 or 2.5.
I hope this was instructive.
Rich B.
(1b1)
There is an easier method to use rather than Soroban's.
Work is inversely proportional to rate.
Thus, if it takes 15 days for 7 men how long for 3 men and x days.
Since we have a inverse proprtion we know,
$\displaystyle 15\times 7=3\times x$
Thus,
$\displaystyle x=35$