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