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<?php
add_filter( 'wp_head' , 'related_products_style' );

function related_products_style() {
	
	if( is_product() ) :
	?>
	<style>
	.woocommerce .related ul.products li.product, .woocommerce .related ul li.product, .woocommerce .upsells.products ul.products li.product, .woocommerce .upsells.products ul li.product, .woocommerce-page .related ul.products li.product, .woocommerce-page .related ul li.product, .woocommerce-page .upsells.products ul.products li.product, .woocommerce-page .upsells.products ul li.product {
		width: 24% !important;
	}
	</style>
	<?php
	endif;
}