Answer: 90 000


Using a small number of revolutions

  Circumference = $\pi\times$60 cm = distance travelled in 1 revolution

    Circumference = $\pi\times$80 cm = distance travelled in 1 revolution

Wheels travel the same total distance

Small wheel:
1 revolution: 60$\pi$ cm
2 revolutions: 120$\pi$ cm
4 revolutions: 240$\pi$ cm

Large wheel:
1 revolution: 80$\pi$ cm
2 revolutions: 160$\pi$ cm
3 revolutions: 240$\pi$ cm

3 revolutions on the large wheel = 4 revolutions on the small wheel

Small wheel: 120 000 revolutions = (4 revolutions)$\times$30 000
                                               = (3 revolutions of large wheel)$\times$ 30 000
                                               = 90 000 revolutions of the large wheel


Using ratio
Ratio of the radii is $3\colon 4$
Ratio of the number of revolutions (to travel the same distance) is $4\colon3$
$\therefore$ larger wheel makes $120000\times\frac{3}{4}= 90000$ revolutions