Let v=number of volleyballs. Cost of v volleyballs=$15v.
Number of footballs=5v. Cost of 5v footballs=5×35v=$175v.
Cost of basketballs=4×15v=$60v. Number of basketballs=60v/25=12v/5.
So total cost=15v+175v+60v=3750, 250v=3750, v, the number of volleyballs,=15.
Number of footballs=5×15=75. Number of basketballs=(12/5)v=12×15/5=36.
Total number sold=15+75+36=126 balls.
CHECK
15×15+36×25+75×35=3750, total cost OK.
Number of footballs=5×number of volleyballs=5×15=75, OK.
Sales from basketballs=36×25=$900, sales from volleyballs=15×15=$225, so sales from basketballs is 4×225=$900, OK.