There was 300 girls and 400,000,001 boys if we timed it together how much will get

I thing you mean 300 and 400,000,001 timesed together (multiplied)

so you could use a calculator

or

400,000,001=400,000,000+1 so

300 times 400,000,001 = (300 times 400,000,000) plus (300 times 1)

400,000,000 times 300 = (3 times 4=12, count the zeros and add them togethter) = 120,000,000,000

300 times 1=300

300+120,000,000,000=120,000,000,300