Chapter 1 - Measurement and Problem Solving - Learning Path Questions and Exercises - Exercises - Page 29: 35

$187515\times 10^{5}$ cells

$1 mL= 1000 mm^{3}$ $250 mL= 250\times 10^{3}mm^{3}$ volume of blood is received in 60 secs. So, in 1 sec volume of blood received is $\frac{250}{60}\times 10^{3}mm^{3}=4.167\times 10^{3} mm^{3}$ Therefore, number of cells received in 1 sec = $4.5\times 4.167 \times 1000$ million = $18751.5 \times 10^{6}=187515\times 10^{5}$ cells.