In an examination, 70% of the candidates passed. If the number of candidates who passed was 350, how many failed?

Problem (1)

Step 1: The container contains an initial volume of $70$ L of solution with $250$ units of solute.

The target total volume is $300$ L.

Step 2: Let $x$ be the volume of pure water to add (in L). The equation is:

$70 + x = 300$

Step 3: Solve for $x$:

$x = 300 - 70$

$x = 230$

The volume of pure water to add is $230$ L. The solute amount remains $250$ units.

230 L

Problem (2)

Step 1: The tap fills at a rate of $1$ L in $10$ s.

The filling rate is:

$r = \frac{1 L}{10 s} = 0.1 \text{ L/s}$

Step 2: Time formula: $t = \frac{V}{r}$ or equivalently $t = 10V$ (in s, where $V$ in L).

For $24$ L:

$t = 10 \times 24 = 240 \text{ s}$

For $40$ L:

$t = 10 \times 40 = 400 \text{ s}$

For $58$ L:

$t = 10 \times 58 = 580 \text{ s}$

For $247$ L:

$t = 10 \times 247 = 2470 \text{ s}$

240 s
400 s
580 s
2470 s