Two spherical balls having equal masses with radius of 5 cm each are thrown upwards along the same vertical direction at an interval of 3s with the same initial velocity of $35 \mathrm{~m} / \mathrm{s}$, then these balls collide at a height of $\_\_\_\_$ m. (Take g = $10 \mathrm{~m} / \mathrm{s}^2$ )