let

b = the number of boys

g = the number of girls

1650 students took part in a parade

b + g = 1650

there were twice as many boys as girls

b = 2g

g = (1/2)b plug into b + g = 1650

b + (1/2)b = 1650

by solving we find

b = 1100 boys

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there were 1100 boys at the parade.