bicycle has 2 wheels and 2 pedals
tricycle has 3 wheels and 2 pedals
EQ 1 : 2b + 2t = 170 pedals
EQ 2: 2b +3t = 206 wheels
subtract EQ 1 from EQ 2
2b +3t = 206 - 2b + 2t = 170 = t=36
there were 36 tricycles
36 x 2 = 72 pedals
170 pedals - 72 = 98 pedals left
98/2 = 49 bicycles
36 Tricycles and 49 bicycles
