expression, godan verb (-ku)
to count on the abacus; to do abacus calculation
Literal meaning: moving the beads of an abacus to perform arithmetic. The verb はじく (弾く) specifically refers to the flicking motion of the beads.
祖父はそろばんを弾くのがとても速い。
My grandfather is very fast at counting on the abacus.
expression, godan verb (-ku)
to calculate cost-benefit; to be guided by self-interest
Figurative extension: to weigh gains and losses, often with a connotation of selfish calculation. Used when someone is thinking only about personal profit or advantage.
彼はいつもそろばんを弾いてから行動する。
He always acts after calculating what's in it for him.
そんなにそろばんを弾いてばかりいると、友達を失うよ。
If you keep calculating your own benefit like that, you'll lose your friends.