Apreschool playground has both bicycles and tricycles. there is a total of 30 seats and 70 wheels. how many bicycles are there? how many tricycles are there?

the solution of this question is in the picture.

check the picture below.

well, the angle at the vertex d is clearly not a right-angle, let's check for the one at the vertex f which does look like,   however if it's indeed, that means ef is perpendicular to df, in which case their slopes will be negative reciprocal of each other.

so the negative reciprocal of the slope of ef is 1, however the slope of df is not 1, is 1/2, so nope, they're not perpendicular lines.