If you prepare a 1/50 dilution of a 75% solution, what is the final concentration of the solution?

.75 / 50 = .75 x 1/50 = .015 = 1.5%

1:1 dilution of 5mg = 1/2 dilution
 = 5mg x 1/2 = 2.5mg/ml

You could dilute 1 ml with 999 ml of another solution,
stir and remove 1ml and repeat the dilution 2 more times.  
This would bring the dilution down to 1 billionth.

If you dilute the solution 1:50, then basically you are adding 50 times the solution volume of solvent alone. so your final concentration of solution is 75/(100+5000)*100 = 1.4%

For 1:1 dilution, 5mg/ml now becomes 5mg/2 ml. so 2.5 mg/ml.

 

A 1:5 dilution of a 75% initial solution is 75 / 5 % = 15%,
so a 1:50 dilution is 1.5%.  The final concentration is 1.5%.

Set up 10 tubes, the first containing neat bacterial suspension with 0.9 cm3 diluent in each of the remaining 9 tubes.  Transfer 0.1 cm3 neat to the second tube and mix. Change pipette tip after each mix.  Continue right to the end.  You will have created a dilution series from 1/10*1 to 1/10*9.  The 10th tube will be your 1/10*9 dilution of bacterial cells as you required.

A 1:1 dilution of a 5mg/ml = 5 / (1 +1) = 5 / 2 = 2.5mg/ml.